Geography 140A Section 4 Lecture Notes

We are moving on now to inter-fluvial regions, the majority of landscapes!

At about 42 he defines different parts of interfluvial landscapes, uplands, hillslopes, valley floors. Over time, river incision will cause the rest of the landscape to feel its effect. The surrounding area will weather and become regolith. Hillslope transport will be felt in the valleys and the uplands. Basically, the rock mass exposed by the river will itself erode and transport downhill. Hillslope processes in the interfluve are 'abundant, pervasive natural hazards'.

Pic of sediment transported as bedload during a flow event

Bedload can move sediment quite a bit during high discharge events!

Why do areas near oceans receive so much precipitation?

Ocean temperature effect--parts of the continents adjacent to the ocean bc the warm ocean surface heats the lower atmosphere, making it unstable and charging it with moisture. Most dramatic in North America.

Bottom of page 48: width to depth ratio of river channels.

Reliant on the strength of the banks and whether or not they can be eroded. If the banks are cohesive with a high value for tau star, theyll be narrow relative to their depth, ratio of 5-10. For erodable banks it will be more like 30-100.

What controls the change of sediment mass in a valley bottom over time?

Inputs--sediment in from upstream, from tributaries, and from hillslopes. Outputs--sediment that flows out. If input>output, accumulation, deposition, aggragation. If input=output, at grade. If input<output, incision, erosion.

What is a discharge hydrograph?

It is a graph of runoff over time at some location, varying due to storm flow, base flow, etc.

Alluvial terrace vs strath vs other valley formation pic

Terraces are a record of tectonism and form systems, generally, but not perfectly.

Q&A terraces

Terraces form at a valley bottom/floor (filled with alluvium or a strath) where a river incises at an increased rate, forming a canyon, widening the area the river incises over time. Two types of terraces depending on type of floor. Renewing incision: runoff event, increased transport capacity, reduce sediment supply, lower base level, increase slope, etc.

What is downhill erosion rate a function of?

The sum of shear stresses Tau, but erosion will not be important until a threshold Tau star is reached. Tau star is dependent on how much the particles jut out--the size of the particles.

Equation for magnitude of surface water stress upon the substrate while flowing down a hill

tau is basically density of water times gravity times depth of water layer times slope of the hill in radians

What is chemical weathering?

"Chemical reaction can break the bonds that hold rocks together causing them to crumble. Water is often a key ingredient to this, as well as warm weather." Most rocks are silicon and oxygen as their dominant constituents, in addition to metal cations at important bonding sites in the structure. Meanwhile, the water has some charged hydrogen ions wandering around, acting as an acid. Other acid sources include CO2 from the decomposition of organic material and the air, forming H2CO3 carbonic acid in conjunction with water, a weak and abundant acid, can form a bicarbonate. Also the organic acids, citric acid, humic acid, sometimes sulfuric. The weathering of rocks by these acid methods depends critically on the climate bc this is related to the amount of water. Also, the presence of biota (decomp and CO2) influences the ability of acids to weather rock as well. Over time, the water could become flushed with sediment, ending the chemical reactions and the weathering, so it has to be flushed regularly. Temperature matters too--hotter rock=more vibrating molecules=easier reaction.

What is DRAINAGE DENSITY???? (Section 4 page 8, will be on exam confirmed asf)(starts at 60min)

*Density sub d = the sum of the length of channels in a basin divided by basin area.* Sigma L over Ab. Total length is all channels of all order added. Equal to the reciprocal of average length between channels (like putting a line over the V a tributary forms). Essentially how many channels are crammed into the landscape. Same area but more channels is a higher drainage density.

Page 19 of section 4. The word 'weathering' is the alteration of rock material at the earth's surface, from physical and chemical processes, producing soil, and thus providing the basis for most biota's existence on the earth. Let's talk more about weathering, pg 20.

*Physical weathering*: a pulverization, just someone whacking on the bedrock with an mf hammer. 55:36, shows an example. If there's water in the fractures within the rock, when the temperature drops to make ice, the ice will expand and jostle the surrounding material; this is called 'ice wedging' and it is a form of physical weathering. More important for loosening than actual fracturing, especially bc it happens over and over again. Ice lens growth is related to this. The cold temperatures will penetrate into the subsurface, expand, and form an ice lens. Unfrozen water from deeper down will be brought up, causing deposits of this warmer water to grow, which will become much more important in the weathering, because they create pressure and potentially fractures. Ice lenses may continue to grow up to huge pressures if it's cold. 1:01:50.

How can a rock mass' strength be tabulated, that is, what are its controls? What is the rock mass strength index?

1. *Intact rock strength*, the compressive strength of the rock, like crystal and granite. 2. *Weathering*: How weathered is the rock? It becomes weaker and gets fractured. 3. *Separation of fractures*: How frequent fractures are, determined by how close they are together in space. 4. *Orientation of fractures*: Parallel fractures to the slope, the normal stress pulls on the fracture. If fractures are perpendicular, the normal strength can't pull on them. 5. *Thickness*: Are they a foot wide or hairlines, inches? 6. *Continuity of fractures*: The more that come together, the greater the fracturing potential. 7. *Outflow of groundwater*: If you see groundwater leaking out, it means there are potential voids/fractures in the subsurface, and weathering along the fracture systems that may carry groundwater. Useful for assigning rock strength at scales of engineering vs just a tiny sample in the lab. Thus, strong rock=steep slopes. El Capitan is unweathered, unfractured. The Dolomite Alps of Italy have carbonite slopes. Canadian Rockies. Magma can penetrate into weaker rock and create differential erosion and stuff like that. In places like the Colorado Plateau, with horiztonally stacked strong layers of smth like sandstone over weak layers of smth like shale, the strong rock can be undermined. This happens when runoff events erode and carve channels into the weaker layers, water will pool at the base of the strong rock, and water will saturate into the strong rock. The rock will collapse as it is undermined, causing an erosion of vertical cliff edges where the strong rock overlies the weak rock, is called cliff propagation or 'sapping', like Monument Valley on the Arizona-Utah border. Falls at steep slope bc of rock strength again!!

What geomorphological conditions let to Hurricane Katrina?

1. Constant point bar wandering of rivers means new sediment and old sediment layered, with different mechanical strengths. 2. Topography of New Orleans, with low relief, hardly 30ft along sea level, with the highest part of the landscape at the natural levies along the rivers. Vast areas of that neighborhood are *below* sea level, but still developed. Water has to get pumped to these below-sea-level areas and levies have to be built on the flats...a perilous situation. A levy over a flat. Pictured map: topographic.

What are the types of acid actions that weather rock material?

1. Dissolution. Like, a sodium molecule would come up from the rock and become an ion in the water; literally analogous what happens to the sugar in sweet tea. Becomes an ion. Not a very powerful process. The solubility depends on the pH and the type of ion, there's a graph in chapter 6 of the book about this shown at 15:53. Calcium carbonate and ferrous oxide (limestone and iron) dissolve quite a bit at normal pH. If the pH changes a lot due to a lot of dissolution or another reason the solubility may change a lot. Happens in volcanic systems bc they leak gases that react to form strong acids in water. Also happens with strip mining bc it exposes sulfur bearing minerals causing sulfuric acid to form and the water gets a low pH, weathering downstream of strip mining sites and causing some gross deposits. 2. Hydrolysis. Involves the substitution of an acid for a metal cation in the mineral structure. Will react with the mineral to kick out something like the potassium cation, forming a clay mineral (19:39). Can potentially cause the formation of something like clays in strong rock mass, and accompanying particulate. 3. Carbonation. That is either of these involving carbonic acid, given a special name because of the abundance of carbon dioxide in the environment. When hydrolysis happens with carbonic acid, there is a lot of bicarbonate in surface waters, HCO3-. If that bicarbonate goes down to the ocean, it gets locked away in the rocks of the earth's crust; an important control on climate (EPS 7).

How do rivers transport sediment?

1. Dissolved solutes can exist because of the same processes that caused physical weathering as discussed earlier. 2. Sediment can get suspended in the water due to flurrying eddies. Qsub ss. 3. Shearing force might take particles along for the ride (at the bottom of river) constituting bedload transport, mass per time being conveyed as Q sub sb. Qsolute=number one, Qs=last two, sediment flux usually includes last two whereas rock flux includes all 3. Page 37 btw.

What controls the amount of sediment in river channels?

1. Supply. Is the channel supply-limited? That would determine Qs. 2. The transport capacity. This is controlled by water discharge, the stresses on the bed (related to Qw), the slope, the size+density of the sediment (finer=easier to keep aloft in sediment transport, rougher=more bedload).

Page 28. Talking about landscapes with regolith on the surface or fractured rock, which are more significant.

1. Earth flows, ductile movements down slope; can be quick or last centuries. If a failure occurs within the rock, failure on the surface accompanies, leaving behind some kind of basin from which the original form was evacuated and some deposit on the landscape. An example at 22:41 of the Colorado Rockies. Can even create a natural dam. Just linear downhill. 2. Rotational slumps, more of a slide than a flow. Happens when the plane is slightly concave upward, separating out the block of material. If there is a failure deep beneath the surface that breaks and the overlying material starts flowing downslope. Creates tension uphill from where it flows, and compression downslope from where it flows, creating kind of a bowl shape. May flow downhill or stay as a little ovular deposit. The formation kind of looks like a toe. Sometimes though they might fluidize and flow downhill. Central CA coast range example. Creates like a staircase of scarps. Oso slump in march of 2014, Washington. 3. Shallow translational slides. Let's say the failure plane and break is closer to the surface, so the overriding material mostly moves on down slope with primarily shear stresses, but stops due to slightly concave upward slope (or how close it is to the surface?). Called translational bc it basically just moves downhill. Often occurs at the regolith and the soil. 4. Debris flows, which are roughly equal mixtures of water and rocked material, sourced as a shallow slide or slump. Because they are so full of water, can flow for miles at high speeds downslope. Leaves deposits on both sides of the channel, debris flow levies, some of the rock material bouncing over the side of its direction. Can incise into bedrock much like rivers transporting sediment can. Looks almost like a river that has no water anymore or something. Take a path that has a 'debris flow fan' at the bottom, a pile of sediment that occurred due to debris flow. Bigger rocks floating within smaller rocks at these deposits, a matrix of smaller rocks and the water fluidizing the material. A very important and significant natural hazard in mountain terrain, like in the Alps. In steep terrains/places where there is a transition from a mountain canyon to developed area, one can just build a retention basin to catch debris flows.

Page 43. When an erosional or depositional regime persists over a long period of time, distinct morphologies result:

1. Erosional regimes will expose the bedrock and incise, becoming a bedrock channel. Characteristics: usually confined by bedrock to a relatively narrow valley, having to rise a lot since it can't spread, potentially causing large floods. An example is the Big Thompson river in Colorado. A house got by a flash flood and like 60 people died... Straths can form in valley bottoms from thin alluvial deposits as floods sink back. The fastest flow of the river channel will pile against the outer bank, creating a pressure gradient that turns the river along a curvy path. This is a greater shear stress at these outer walls, creating erosion that causes the curves to grow, and these are called meanders/meander bends. Just a few milimeters per year. Goose Necks of the San Juan. Never purely bedrock exactly, get bursts of sediment from rock falls, slides, etc, having to sweep that sediment downstream; some alluvial sections. Pictures of meander bends: 2. Alluvial channels results from the buildup of depositional channels. The channels are floating entirely on sediment and tend to be very erodable. At low slopes, only gently curve, aren't exactly meandering though. Example is the Coastal Plain in Georgia; maintain this linear form bc the stresses are low at low slopes. But then, if the slope is sufficiently large, the meander stresses form in these channels too, with deposition on the inside of the bend. Meander bends are more common with alluvial channels. Outer park is a cut bank, depositing is a point bar. Development of these meanders continues and continues over time. Curvature can be crazy. A dynamic process that can erode meters per year, but not permanent. Oxbow lakes form from 'avulsion', a change of the river channel form bc of overbank flooding causing erosion and redirection of a downslope alluvial channel. Magnitude of wiggliness is sinuousity, calculated as the ratio of length of channel to valley length, meaning the length between curves. Sinuousities can be around 3, but as high as 10-15, as low as 1.5. Has a tendency to increase because of meander bends that is countered by avulsion over time. Major problems for development, property, and political boundaries, like Illinois, bc the state boundary reflected older meanders. Examples: Red River in Oklahoma. Best example is Mississippi river in Southeast USA.

What are the basic forms of mass wastage?

1. Flow, where the straining is distributed with depth, behaving like a smooth deformation at big scales. Stress profile looks like velocity profile of rivers. 2. Slide, like a book sliding down the table bc it was pushed to be displaced from its surface! 3. Churn, small-scale motions that are very irregular, processes agitating the material near the surface, particles kind of moving like eddies. 4. Heave, cyclic churn--a particle going up and down and up and down, like when ice lenses grow during cold night, pushing up water with freezing and it flowing back down when it melts. Also something like burrowing rodents can create churn, pushing material around.

What gives rise to depositional channels?

1. Large input from surroundings, ie hillslope transport and channel input, transport capacity remaining equal. 2. Downstream reduction in transport capacity and therefore outflow.

Landscapes with diff drainage densities

1. Low--southern alte plano in Argentina 2. Low bc of vegetative density 3. Rockies, medium drainage density 4. Southern alps of NZ--high drainage density. Lots of runoff, very steep slopes. Less below timber line bc of infiltration rates into forest soil and vegetation cover. 5. Badlands--very high drainage density. 6. PA coalmines show how a smooth hillslope will get cut into.

Pg 25, almost certainly on the exam. What are the four stages of a weathering rock, or the standard progression of soils?

1. Rock, silicates with lots of metal cations. At this stage, nutrients are not accessible to biota. 2. Partial removal of metal cations, the more mobile ones, to form clays. Common clays are illite, smectite, and vermiculite. Happy biota, nutrients are accessible. 3. Removal of most silicates and most metals except for iron and aluminum; iron and aluminum oxides and hydroxide form, plus kaolinite clay. This represents a serious degradation of the nutrients for the biota; for organisms where this regolith dominates, they must find other ways to access nutrients. 4. Removal of most of the silicon and the iron, leaving aluminum oxides and hydroxides, known as gibbsite. These are nutrient disasters from a biota perspective,

What factors contribute to channel roughness?

1. Small scale topography on the bed. a. Grains, like are they boulders or pebbles? This is akin to z not in the law of the wall. b. Bed forms, that is, the organization of sediment into dunes or ripples, and how much those little humps stick up. In the z not case, this local roughness is approximately the diameter of the grains or the dunes divided by 30. 2. Large scale irregularities in the channels itself. Its curvature vertically (1:07:01) may restrain the flow. The more 'deeps' and 'shallows' there are, the more channel roughness, and the higher Manning's n. 3. Vegetation: if there are water plants growing on the banks of the channel or branches from trees or logs, that increases roughness. The value for n is a quantity you can learn measuring slope and depth, and the USGS has visually tabulated it. 1:10:32 shows variation in Manning's n for different landscapes. In the pic shown at 1:12:04, he said you can see the white rapids as an indication of a disruption of flow, hence a high Manning's n. A placid stream at 1:13:36, low Manning's n.

Page 38. Two notes about suspended sediment flux:

1. What is available in surrounding hillslopes is important--if there is fine-grain regolith, the suspended sediment concentration can raise a lot, but if there's bedrock, the suspended sediment won't be as proportional to sediment load. 2. Sometimes at areas of high relief, gravel and softer rocks like clay are moving along the bed with these rocks in suspension. In the lowlands, we'll transition from gravel to sand going upstream to downstream, highland to lowland. Important for riparian ecology: salmon have to spawn up by the gravel, rather than downstream where the bedload is constantly moving.

Summary of depositional/erosional conditions for rivers:

1. What is the supply of sediment? Is it large or negligible? 2. What is the sediment transport capacity? It creates deposition if downstream. Panamint Range in Death Valley CA shows an alluvial fan incised with a new channel, maybe caused by an increase in the ability of a river to incise by adding discharge, or a reduction in available sediment. In the Western Plains, the Ogalala aquifer is an overlapping set of sediments that store water and have seen pyrogenic uplift for about 2 million years that have somewhat eroded the aquifer. The Pacos River in TX, and eastern-central MX, incised and cut a valley through previous deposits due to change in slope from pyrogenic uplift, a 'headless' set of plains, the Yano Estacado, deposits with no mountains. Also a significant escarpment.

The case of a structure built on a hill. What are the implications of building a structure on a steep hillslope?

1. You have to build a flat platform for the house (excavate and fill), making the area on either side steeper or more flat, making it more prone to failure. 2. Weight of the house might stabilize the slope if it is steep and not-super steep. 3. Concentrated runoff from the roof can saturate and destabilize the slope. Leakage from underground pipes or maybe nearby farms.

What are the potential problems of this analysis?

1. You need a really long period of observation to see the tails of the distribution, like in the case of a hundred-year flood. It is rare for that to exist, so it is modeled (though contentiously), or you can try to find geomorphological evidence of past floods. Like, when overbank flooding occurs, water will also flow up to the tributaries and get blocked with sediment, called a backwater deposit, finding a record of large floods beyond the potential historical collection record. 2. In either extrapolation case, this type of modeling only works if runoff generation mechanisms have remained the same over time, but in cases like urbanization or vegetation or climate change right now, we just don't know. Climate change has imploded a lot of the civic engineering work trying to figure out these distributions.

Page 13 of section 4. This is what he really wanted to talk about today. There's a water discharge determined by what happens upstream, which has to be accommodated by flow, so how does the channel accommodate it? How do we calculate hydraulic radius?

37:30 diagram. If we have an area of the channel in contact with the ground, Ac, and the velocity of the water itself, U bar, then the product of those two things has to be Qw for that area. Therefore, more discharge has to show itself as faster flow or a larger channel area (level of water changing). Those things will usually be happening simultaneously. If channel width is W and depth is H, then a change in Ac means a change in either. Hydraulic radius is the perimeter of this wetted cross-section times the area and usually equivalent to depth for large channels.

What are the variations of the characteristic longitudinal profile?

A bedrock area at the transition point will force steepness bc it can't incise and can't become alluvial due to a lack of erosion. Alluvial rivers may get steeper where deposition occurs from sediment deposits from tributaries.

How does the characteristic longitudinal profile develop? What might trigger incision?

A change in the watershed allowing for more incision to happen can be considered a non-progressive event. maybe a deposition. Also tectonic changes, 'progressive' Five ways incision might be triggered: 1. increase of average discharge (climate change or vegetation or land-use changes) or peak discharges (hillslope hydrology or climate change) 2. decrease of sediment load (climate change, vegetation change, land-use change) 3. change in uplift rate that increases slope (pyrolift texas) 4. change in grain size of sediment load 5. base level lowering. These could occur for either incision relationship, including alluvial's transport capacity/supply. 23:01 more depth. To 32 when I started listening. à incision will abandon the old fluvial valley floor and leave it high and dry as a nearly-flat surface = a Terrace * they may be alluvial (abandoned floodplains) * or in bedrock (strath) with a thin veneer of alluvial deposits * can be many terrace levels in a valley ß cyclic climate change, for example Terraces are tremendously important geographic features: * flat surfaces, near rivers, immune from flooding à excellent locations for villages and cities * good transportation paths * record history of major environmental or tectonic changes Terraces are historic sites, good road networks, recording a change in the landscape as a fossil of an earlier version of the longitudinal profile of the river network. Pictures.

How does expansion and contraction cause weathering?

A rock consists of different mineral grains with different compositions within the rock, and they'll expand and contract causing stresses within the rock. Can get wet or dry, cool or heat up naturally, maybe heat up due to forest fires, which will cause differential expansion/contraction and therefore loosening.

How does exfoliation cause weathering?

A rock under a bunch of material would form under immense compressive stress. As erosion takes away the overlying material, the vertical and some horizontal forces are removed. The rock will expand vertically, but not horizontally, where it is still surrounded by rock. It will expand vertically, creating forces parallel to the surface; the fracturing will be parallel to the surface as well. It looks like layered potato slices or something. In the granites of the Sierra Nevada, you see a lot fo exfoliation, because the rock overlying it has to be homogenous and strong, so it rockets up along this newly formed channel.

How does cohesion manifest?

A zone between the mathematically calculated tau star assuming no vegetation and the actual point of stress force where the erosion will start to occur.

What are the forms that *drainage networks can* take?

A. *Dendritic*. If the underlying rock is homogenous, no real variation, there will be no actual pattern in the orientation of the various channels. It'll make a dendritic network and look kinda like veins, but not be super symmetrical. Good example is western flank of the Appalachian plateaus and the Ozark plateaus. Northwestern AL, Mississippi border region. Examples are being shown around 30 mins. He said he's gonna show us a map and ask qs abt it on test. B. *Fracture line*. If there is a part of the substrate that is particularly fractured or weak (zone of fracture), the dendritic networks might feed into, but ultimately follow, this line of weak substrate. An example he showed us earlier in the semester was the CA coastal range/the San Andreas fault, Scotland and the Great Glen Fault, Valley of the Kern River/Kern Fault. C. *Rectilinear.* There are many places with zones of fracture that are larger, and fractured along joints of tension. They'll intersect in sets. You'll see a rectilinear network of channels--not necessarily orthogonal, but usually straight. Southern Central Utah. D. A *radial* network needs a highland, and thus the channels will flow down. E. *Trellis* networks--sometimes, there are longer strips of land with a higher Ks across the landscape. A stretched out version of a rectilinear network. Good example is Appalachians--go long way along weak rock until they abruptly cut through weaker rock.

What causes badlands?

A. A high Ks--susceptible substrate that it's easier to carve into. B. Sparse vegetation (semi-arid or arid), caused by natural factors or human interference like strip mining. C. A small infiltrating fraction of infiltrating water, since surface runoff is generally more effective in eroding the substrate. D. Slope--from some larger sale topographic gradient. Something like thrust-fold formation. In South Dakota it's derived from the White River incising down into the landscape.

Why is this possible, even though water is so much weaker than rock material? How can rivers get so deep into the rock, like they're a dumb little hydrological saw??? (btw, he's explicitly talking abt downhill flows of water I think, based on 1:06 ish)

A. Abrasion, basically a 'process of sand blasting'. In the water, there are tiny little particles vibing in the water that collide with the substrate and causing the underlying rock to come loose. Produce a 'polished, but scalloped' surface. Once the abrasion starts, the collision will be concentrated in certain areas, redirecting the water flow, which is why we see lil divots. B. Plucking/quarrying--like we saw earlier, when this river is flowing downhill and encounters rock material jutting up, it will push on that piece of material. A large normal stress produced at that site may cause a fracture, which will separate a lump of rock that will get hauled away by the flow (especially important when there are fractures/etc that cause the rock to be weak, and most rocks are like this to varying degrees) C. Cavitation looks like abrasion, but is caused by the impact of collapsing bubbles against the surface of the rock. Water flowing quickly will create a low pressure zone, and gas bubbles will form from irregularities on the surface. When they collapse, it will be powerful and rapid enough to fracture the rock--the speed here causes the water to act almost like a solid bc it has a relatively significant viscosity. Damages the propellers of ships and makes submarines loud!!!

What happens when there is snow melt or rain delivered to a hillslope?

A. Diff pathways the water can take--will flow off over the surface, or go down to the subsurface, which will go into a deep water reservoir or come back up to the surface on the hillslope. B. The volume of water flowing down the hillslope increases downhill, bc the hillside acts like a giant catch-basin; it is the cumulative effect of water (maybe represented by an integral). C. The water that is running off the surface and onto the ground produces forces and stresses on the ground, related to 1. surface water runoff, which will exert stress upon the substrate 2. seepage pressure from water that flowed into the ground and it now flowing up and out to the surface, pushing on the particles in the soil/rock fragment and exerting seepage pressure, which manifests as stress. D. These stresses can, but will not necessarily, cause erosion.

How does downhill runoff potentially cause erosion?

A. Illustrated by 28:35 in lecture, there is a shearing force exerted by the water onto irregularities of the surface rock (parts that jut out). The integral of these shears is the stress tau--sum of pushes whose rates may change a bit. When that stress is sufficiently large, erosion will happen. The erosion rate is a function of Tau.

What does the discharge hydrograph express?

A. Variability of wetness in the channels B. Magnitude of floods C. something else he said that i missed

What is the positive feedback loop of incision that causes stream capture? How does this lead to drainage network/drainage basin/watershed?

Among rivers close to one another, there is going to be a difference in their incision rates, meaning a deeper valley, and a steepening of their tributaries. Because of the incision rate's dependence on slope, once a valley is deepened, tributaries will deepen and expand headward, aka they'll spawn their own tributaries. More tribuatries means more area to catch runoff, therefore more discharge, therefore more incision. This means the stream with the initial advantage will eventually deepen its advantage, and then expand such that nearby channels coalesce into a larger channel network. It's called a *drainage basin/watershed* because eventually these various tributaries have a common point of runoff, and it's why things that happen upstream end up affecting the entire drainage network.

What is a natural levee and why do they form?

An alluvial river that overbank floods will deposit heavier, coarser minerals first, then the finer minerals fan out below, making ridges or 'natural levies' that line the channel. In the case of somewhere like the Mississippi, the tributaries will encounter the natural levy system, Yazoo rivers, and the Yazoo basin (pictured). The river may also lose transport capacity as it smooths out and flows toward the ocean, which could case buildup in the river, causing its levy to rise about these natural levies, and divert, so evulsion occurs, happening repeatedly in the lower Mississippi Valley.

Biotic activity is also a factor in weathering!

Animal scratching, etc.

In what landscapes does one find braided river channels?

Areas with high slopes, high sediment supply. Places where banks are eroded, because weak banks leads to wide channels/shallow channels, which lessens transport capacity, so they tend to braid when overwhelmed by sediment vs adequately moving it to to meander shear stresses. Landscapes with mountains and alluvial rivers: New Zealand, downstream of glaciers and ice sheets that have scoured their beds and overwhelming rivers like in Greenland, areas where the discharge hydrograph is variable meaning normal transport capacity may overwhelm the river (happens in the desert). Because they are changeable, they are difficult for engineers to deal with.

In most applications, we want to understand the average discharge in the cross section. How do we do so through Manning's relationship?

At every location along the cross section, there is a vertical profile related to the bed's roughness and the stress on the bed. It becomes complicated once we introduce the sides of the channels. Instead, understanding average velocity can be done through a more simple, empirical equation. Page 15 of sec 4. How fast the water flows on average across the cross section is equal to average depth times the slope times some number, the roughness of the channel. That roughness, n, is 'Manning's Roughness'. U = H2/3 S1/2 / n. If there is an introduction of new water upstream, the accommodation may occur by increasing velocity, which can occur because of increases in depth or maybe slope. They are attached to this factor of 'channel roughness'.

At 1:00:10 and chapter 6, the categories are plotted around the world.

At the mid-latitudes and western Europe, you see a lot of clays and soils, hence areas favorable for growing, but it is contingent on temperate climate and precipitation. Southeast USA and tropical Africa, India, and SA are largely stage 3 and nutrients are not as abundant, but can be nutrient-enriched through stuff like atmospheric transport. In the Amazon and Indonesia, the soil is more like stage 4. A lot of stage 1 areas are arid as hell, like the deserts. We can also imagine soils down beneath that are nutrient-rich and the solid rock beneath it bc of downhill piling and transport. Mid latitudes are sort of a happy medium.

How is channel area Ac (depth and channel width) related to velocity of water flow U bar?

Average rate of flow tends to increase with the shear stress of water flow against the bed; shear stresses of water flow against the bed rock, tau b, increases roughly proportionate to velocit squared. Over a large average, bed stress will be related to depth of the water; the more flows, the more interaction forces between the water flow and the substrate. So, as the depth of water increases, the velocity increases as well. In a discharge hydrograph, as Qw rises due to a storm flow period, more water in the channel means more depth of water, leading to an increase in velocity. At the same time, there will probably be an increase in width, and this is the morphological accommodation of the increase in flow. Water discharge is usually determined by what is upstream, so the causal relationship means the changes in W and H and U bar are due to a change in Qw.

Moving to page 40. Notice a big discharge event will carry a lot of sediment, whereas middle and low discharges hardly carry any. So, what discharge event is associated with the most sediment transport--the magnitude frequency question. What are magnitude frequency analyses and how do they help us determine the most important runoff events?

Basically, are a bunch of little events more impactful than more-sparse big events? We know: As the discharge hydrograph has stormflow peaks and base flow falls, we know Qw drives sediment flux. Answering the magnitude-frequency question: Convert the discharge hydrograph to a frequency graph like we did for 'hundred year' floods. Duration of which a river conveys a given discharge, Qw. Use the sediment flux relationship, showing us there is no relationship bt Qw and sediment flux until a higher threshold is reached. So, we should multiply these curves to find mass transported by these flows over a given time period, in terms of discharge value Qw. That is, the integral of these two gives us mass per discharge! The integral of this is the total mass moved over time. The peak of this graph will give us Qw star, the most important morphological changes. We can consider this analysis in cases like the urbanization of a channel area, where runoff generation is effected and perhaps made more unstable, causing the frequency graph to shift toward more volume-heavy events, meaning more mass per time transport occurs, and the Qw leading to an increase in the volume of important discharge events for sediment transport. Applicable to many other geomorphological problems, like a consequence and frequency analysis. Arid landscapes with higher peak stormflows will have smoother, longer-tail frequency graphs vs humid climates; this type of analysis can help use to compare spaces.

Why is the suspended sediment (#2 in list) of key importance?

Bedload transport is generally much less volumetrically important than sediment transport in terms of the sediment yield. The concentration of the sediment relative to the bed is proportional to the velocity profile for rivers above rock beds, aka homogenized near the top, generally because the sediment can more easily and vigorously be disturbed near the bottom of a water column. Integrating velocity per water, U(z) m/s/waterm^3 and the level of suspended sediment, C(z) rockm^3/waterm^3, relative to a water column will give us the sediment flux term Qss, a function of the velocity profile and hence the rock bed particle sizes, the rate of water flow. I think I'd explain this like since flux is given volume per unit of time over a unit of area, multiplying Uz and Cz gives you For example: If the discharge rises because of a stormflow event, accommodated through water in the channel, hence raising velocity because of Manning's equation. The suspended sediment volume will increase, may bc of mass wastage/sheetwash but also bc of river function. Hence, you can get very large increases in the suspended sediment flux. *Sediment flex is proportional to the water discharge*, usually a constant between 1.5 and 4. River in the Andes almost red with suspended sediment.

Mescalero Escarpment, Ogalala Aquifer (important source of pumped groundwater in this region) and the Pacos River on reiss map:

Being used, mined...inevitably it will be depleted and devastate the economy of the western high plains :/

What is the law of the wall?

Calling magnitude of velocity Uz (z being depth of water to the bed), the 'law of the wall' says: Uz= (2.5)/(square root of density of the water)*(square root of bed stress)*(natural log of height from the bed Z divided by Z-not) That relationship describes the rapid change in velocity near the bed that evens out as you rise. Shear stress is proportional to depth and slope, so as thickness of the water in the channel (like storm flow) increases, the entire curve shifts, changing the velocity profile so there is more flow, especially near the bed. Hence the storm flow is accomodated. All else being equal, the steeper and thicker a river is, the faster the flow.

What is the manifestation(s) of erosion associated with the seepage forces?

Causes materials to break off, causing something like a landslide, or the paths that the water is taking will carve out tunnels, causing more erosion to happen (kinda fun kinda fresh I prefer that hahahahah)

The central valley of CA is also a fluvial depositional river, Sacramento and San Joaquin rivers losing transport capacity downstream.

Central Valley is two big alluvial fans coming together, but in opposite directions.

What are some typical magnitudes of the rate of flow of rivers?

Consider a lowland low-slope, wide river, and a mountainous steep, narrow river. For the low-slope river, H=5m, slope=10^-4, n=.04. For the steep river, H=1m, S=10^-2, n is much higher bc it is rougher, about .1. We can calculate from these values the average velocity over a section of the channel. A typical rate of flow for a river is about 1 meter per second, and we can see that the variation is not very different between these two cases. You'll never be too wrong if you guess about 1m/s.

What are deep-seated landslides, and how do they potentially cause flooding events after they occur?

Deep-seated landslides occur regardless of regolith or bedrock material in landscapes with high relief (1-3km) and a steep slope. Can cause a fraction of relief of the entire mountain very quickly, spreading out on the valley floor. Often forms landslide dams if there is a river below, and these lakes impounded by the landslide dams can erode the deposit and cause a flood by the lake damming downstream. Not very common, but enormous features. Examples: Swiss Alps. Two of these occured in the Tetons/Western Wyoming, one by the Montana border, one east by the Grovant slide. Caused almost the entire relief of the valley and had a 100m scarp. Large rocks pile on top of fine particles. It impounded a lake that rapidly drained in 1927. Madison River Canyon Slide in the west caused a deposit that formed a slope due to degree; 25 people died bc of a campground down there. Triggered by an earthquake, adding stresses at potential failure planes. In Frank, Alberta, Canada, in the Canadian Rockies, a railroad and highway were built after the slide. The British Columbia area also had one. Tough to figure out what parts of hillslopes are scars from deep seated landslides. Examples of NZ slide and the slide that triggered the Mount Saint Helens eruption.

Thresholds of channelization vary bt hillslopes why?

Differences in temperature, water, vegetation, cohesion, rock material size, the steepness of our slope (S in tau)

What might cause a difference in drainage density? What might cause a landscape to change its drainage density?

Erosion starts at the threshold of channelization, where driving stresses/forces overcome the resistance of the rock material. Anything that makes that threshold occur higher up the slope will lead to a higher drainage density. A landscape might be converted from low to high if seepage causes a landslide? Or a change in land use? 1:10:41 1. *increase in driving forces.* One way might be climate change--more intense storms create more runoff w/o more vegetation, so more discharge. Reductions in infiltrating fraction, so more surface runoff. This can occur if you do something like introducing cows, structures, or pavement, which will press on the land and allow less seepage. 2. *or decrease in resistance/cohesion.* Examples include thinning of vegetation as in land use changes or climate drying, hence why they're higher in arid lands--less vegetative cohesion.

What are some figures for sediment flux annually?

Figures in image.

What are typical river incision rates?

For *actively incising rivers*, rates range from 10^-1 millimetres per year to one millimetre per year. You can get a range of 1mm-5mm per year in terrains of high relief, with high contact rivers. Lower end of the scale is landscapes with more gentle relief (but aren't flat per se). Actually a quite immense ability to shape the landscape, especially in mountain ranges.

How fast is a river able to incise downward?

For an *alluvial* channel: The same thing, but incision rate is related to ratio of transport capacity to the sediment supply. For a bedrock river it's dependent on stresses on the bed aka slope, toughness of substrate/potential for erosion (ks), discharge Qw, C as an 'abrasions and quarrying', or 'the river is depositional' coefficient depending on the sediment flux.

What types of landscapes produce lots of sediment yield?

Generally, conditions of high relief and wet conditions. Sediment yield, not solute, will generally carry more of the total load. However, some rivers like the St. Lawrence have entirely solute load because the Great Lakes upstream take all the sediment. A lot of weathering like calcium, magnesium, potassium. From half air and half rock is sodium, chlorine. Sulfate is from sulfur minerals. Generally from the air is bicarbonate, partially from weathering and partially from precipitation delivering biogenic gases and ocean spray in the form of sulfates.

Let us think more about how rivers convey runoff. How is it that river channels convey water? How does Qw relate to the shape of the channel, and how does it vary at various points?

He sketches it at 14:50. Water delivered to watersheds has to balance its mass with what is present in the channel, hence our earlier equation Qw=Area of basin - (precipitation - evaporation aka runoff rate) - changes in storage. The changes in storage term is what becomes operative studying stormflow peaks, because the groundwater will be either accumulating or depleting. The landscape has to 'deal' with this mass of water, in terms of how fast the water is flowing and the shape of the channel itself, so one knows that the channel will be shaped by the flow and have to accomodate it. There are going to be large variations from place to place in a basin in this Qw term, but as we average over time, storage changes start to matter less.

Factors that make the threshold of channelization occur near a ridge crest will increase drainage density. Now he wants to discuss the 'badlands' (not useful for agriculture); what are they?

High drainage density areas with no soil cover--South Dakota/Sioux territory as an example. Drainage density is extremely high; channelization is intricate. Death Valley's Zabriski (sic?) point.

How can RGM vary according to landscape coverage?

If there's some region of the watershed that is forested and then urbanized, you'll see more overland flow, hence moe rapid runoff and stormflow peaks downstream. Or if part of the landscape becomes cropland, there is less infiltration, and more overland flow.

Last time, he discussed the law of the wall (velocity variance at a single location) and Manning's equation (average velocity at a given place in a river). How discharge at a location --> Qw=WHU. Page 15 of section 4. Let's look at basin scale variation.

If you look at a drainage network, as one observes the discharge going downstream, it typically increases a lot bc the collection area increases (in general). How is discharge accomodated downstream in terms of these 3 factors?--> all 3 increase, but the width generally increases quite a bit. So, as Qw increases, W^0.5, H^.4, so the account for .9 of the possible variation. Thus, we see downstream the channels get wider and deeper. There is a tendency for large channels to flow faster, but it is a weak dependence, because as water heads down the basin, depth increases, so the channel becomes smoother, but then the slope magnitude will go down; therefore velocity is much more weakly dependent on discharge.

Page 25, a related issue of great importance. How thick is the cover of regolith on a landscape surface, and what are the controls on that?

Imagine the crest of a ridge with bedrock underneath, and regolith is being produced by physical and chemical weathering, starting to mantle the surface, hence ending up with soil columns, which usually end up being about 1m deep. This is because downslope transport occurs as regolith is produced, like water flowing downhill, or landslides. The relationship between regolith production and downslope transport determines thickness level. *Transport limited*: Production>>transport. If there is a strong weathering force+powerful regolith production and very little downslope transport, we see thick regolith. Tend to be more smooth and vegetated. *Production limited*: Transport>>production, you see a lot of exposed bedrock and accumulations of sediment in valley bottoms, etc. Tend to be sharper and less vegetated. *Production=landscape*: Patchy cover of soil on land surface, with a thin regolith cover and occasional exposure of barren rock. Intermediate/mixed landscape at 1:15:10, much of the American west. Transport-limited landscapes exist with abundant production--a lot of water/temperature/less vegetated cover, and flatter ground in general. In arid landscapes, vegetation thins out, channels and carve a less-vegetated landscape more easily, creates a double bind of production and transportation in some cases, bc hillslope transportation is increased. Arches National Park is a big old production limited landscape. Can also occur in areas that were 'scraped' by glaciers, especially if they remain cold and rather dry, easy to wash any regolith downslope with any rain. Cascades is an example.

How do landscapes differ based on their drainage density?

In a low drainage density network, runoff has to travel further to get into the channel network, since channels are so far apart. Likely to be smoother land bt inter-fluvial channels, but in a higher drainage density network, you're gonna run into a channel ever few metres.

A complication to magnitude frequency analysis:

In terms of analyzing magnitude frequency for sediment transport, we have to consider that the sediment has to be available. We should distinguish between rivers that receive a lot of sediment and rivers that do not. Let's call the sediment transport capacity of a river Qc, the maximum mass of sediment that can be transported per unit time by the channel at a given location with a given water discharge. The sediment flux if the actual presence of sediment were not limiting, a max flux! I dot=sediment supply rate=input flux of sediment due to mass wastage and sediment delivery events. Imagine I dot coming in on one side of a watershed. Then imagine no sediment coming in elsewhere. The river's transportation of sediment is Qs. As I dot increases, Qs will increase until the sediment transport capacity is reached and the river clogs up. Divides river-y world into two sort of regimes, where Discharge<sediment transport capacity, called 'supply limited' or 'sediment starved' river, where there is a lot of discharge, but no bedrock yielding rock particles frequently where the water flows fast. In between runoff events these are especially common. Discharge=sediment transport capacity is also called transport limited.

Why, on a geographical scale, are there some places where rivers are incising quickly, and how do we model it? (7:16) How do we know/derive incision rate?

Incision rate (i dot) is the product of several things: 1. *S=slope of the river* 2. *Qw=discharge of the water* measured in volume per unit time, or time per width of the channels. We have reason to think these factors matter: since the stress exerted on the bedrock by the overlying water is represented by Tau=H (depth) * S (slope), we can reasonably infer that, all else ceteris paribus, an increase in one of these things will increase the incising power of the river and thus the rate. Also, the more water there is moving down the channel, the more particles, the more abrasion, so that's another reason why discharge matters. 3. *Ks=susceptibility to erosion*, meaning the mechanical strength of the bedrock, the preexisting fractures in the bed. Ex: high susceptibility means little cementation, like volcanic deposits. Orders of magnitude variation between values for Ks. 4. (important) *C=presence of sediment--index of coverage and tools*, meaning suppose there's no sediment. That means there can't be abrasion! (15:06 shows I dot and sediment as a graph).

On page 23 and 24, we discuss rocks that are the most susceptible to weathering.

Iron and aluminum take longer to remove, with calcium and magnesium on the other side, potassium and sodium in the middle. If there is regolith over time that weathers, it will lose most of these very important nutrients, which then go back into the soil. Shoutout biota.

Page 17 of section 4. What are flood recurrence intervals?

It is the average interval in years between flood discharge events equalling or exceeding a given magnitude. Used in legal documents, legislation, news. A 'ten' or 'twenty' year flood is this sort of thing. A frequency distribution of flood events is shown at 25:01. Integrating this frequency distribution is the fraction of years with annual maximum floods greater than some number, f. The recurrence interval is 1/f, how often floods of magnitude f occur. Examples given at 27:45. If f=.01, it recurs every 100 years, or is the 100 year flood. f=1/2 is the 2 year flood; you can then see how large the floods are for a given recurrence interval. You can then calculate the height of the water for floods at a given year interval.

What is z not in the law of the wall?

It is the roughness of the bed. What it does in terms of flow is interfere with smooth shearing, hence slowing down the flow, so the velocity profile will be more narrow. Since we know that the introduction of flow upstream is what causes many of these changes, what we will see if there is a large runoff event is a rough bed corresponding with a deeper river. The velocity profile will be narrower due to the rough bed, and hence a larger flow will result in a water level rise instead of a flatter, faster velocity profile at the same depth. In the smooth bed case, it would accommodate without a rise more easily. Z not can refer to rocks and boulders, sediment, or irregular bedrock.

The amount of water that the channel network has to deal with is influenced by basin area, which increases downstream with each tributary junction. How does area vary with distance downstream?

It will not only increase, but jump with each tributary junction, and therefore the discharge will increase as well. The graph looks like a set of stairs. River discharges are largest with the highest runoff rates and the largest basin areas, so we can use the map of runoff rates to determine where discharge might be largest by then inferring where the biggest basins are. The largest discharges are therefore the Amazon and the Congo basin (equatorial region). Comparing discharge of these two rivers with normal rivers, the Amazon has Qw of 6300km cubed a year (average), and the Congo has only about 20% of the Qw of the Amazon. Climate change has a varying effect on these discharge volumes. Third largest river by discharge is the Orinoco in South America, a testament to the speed of runoff rate in SA, 17% of Amazon. The Ganges (15%) is 4 and has big runoff rates bc of the monsoons, followed by the Yangtze, 14% of Amazon (now the cyclonic belt rivers) the Mississippi 9% the Yennesse and Yelena 6%; these rankings do vary year to year.

So what does this mean for water flowing downhill?

It will, on this planet, usually concentrate into channels, not sheets. And because of the tendency for channels to form where there is sufficient discharge to cause erosion--downhill--the hillslope will have an upper zone with no channels and a lower zone with channels!!

Section 4 page 10. What's a discharge hydrograph?

Let us consider a watershed being drained by a drainage network. All of the discharge that passes out from its main point is derived from anothe region of hte landscape, the watershed. If we measure Qw coming out from the head of this watershed, it isn't constant, and if we made a graph of it we'd find periods of low flow punctuated by periods of high flow. That graph is what the term means.

What's the relationship between rivers' elevation and distance? What does this elevation profile mean for bedrock and alluvial rivers?

Let's say initial ratio is S 10^-2 and increases to S 10^-4. We see a steadily decreasing slope. What would this elevation profile look like? We know for bedrock areas, Incision = C ks Qw S, and slope decreases, whereas downstream discharge Qw increases. The maximum incision will be somewhere in the middle; it looks upper concave. In the regions where the slope is reduced (lower down), the valley may become alluvial as Qs decreases, maybe a fill, or just a strath.

What about RGM and the network topology (dendritic, etc)?

Let's say there are two basins, one more linear, and one a dendritic network. What does the runoff look like in both of those landscapes during something like a stormflow event? We must ask how long it takes the water to traverse the channel network itself, given where it enters in the basin. Basin linear: The discharge hydrograph will be kind of spread out over time, because there is no particular preference for short or far-away entry points for the water in the system. A flatter hydrograph. Basin dendritic: There is a fairly large zone that is approximately the same distance from the outlet, so water will reach the outlet more quickly.

Pictures!!!

Like minutes 50-55. Arable climates have diff channelizations! Some seepage forces will cause little landslides that remove cohesion and kickstart channelization.

His explanation of high pressure preventing the formation of bubbles actually helps w parts of the beginning of the class

Magma and buoyant forces, etc.

32:57 we have a map of USA rivers by discharge.

Mississippi River is dominant in terms of total discharge, but also the Columbia River. The population growth in the Southeast USA is quite rapid, but there is almost no overland flow and no rivers, hence they are mining groundwater. Amount of runoff naturally available in CA barely supplies what we have...hence desalination.

Most likely new evulsion for Mississippi river:

Morganza spillway. Supposed to keep the MI river from adopting a new course after a flood event in 2011. Civil engineering nightmare uwu. River might have to be dredged out occasionally.

Map of runoff by latitude (PG 2 SECTION 4)

Polar regions--very little runoff. topographic effects are superimposed on top of this latitudinal pattern.

What is the approx equation for erosion threshold Tau star?

Proportional to density of rock minus density of water minus the diameter of the particles (size of irregularity). Important for sediment moving in rivers.

What increases the rate of weathering and more late-stage soils? What increases weathering duration? What

Rate: High temperature, the amount of biota on the surface bc of their source as acid and chelating compounds, and precipitation. Duration: is it an old or new geological surface? Some places in Aus have almost no downhill slope and have been weathered for tens of millions of years, versus the midwestern US, just exposed by de-glaciation 15,000 years ago. The rate of removal is tied to the steepness of the terrain/speed of erosion. In steep hillslopes, you won't really get to the later stages.

Q&A: Law of the wall and Manning's relationship clarification.

Rougher bed=larger velocity profile, the size of the sediment that lies in the channel bed. Five things to understand abt this: Sq root dependence of depth and slope. Natural log a result of roughness. Smooth bed=faster flow, all else being made equal. Most man made drainage systems are rough, like rivers are rough.

What (positive) feedback loop develops during these processes of surface water discharge?

Since either of these types of flows (surface/seepage) can cause erosion, we see the following: water will move in irregular sheets for a while until erosion starts. Then it will create a little channel, and more erosion will deepen and widen the little channel. The presence of the channel will capture more water--true w seepage or surface flow--so producing a larger collection area. So there are more stress forces on the rock material.

Mass wastage events are associated with heavy precipitation especially after seasons of water creating a saturating condition. What might happen if climate change makes the climate wetter somewhere like here in CA?

Slopes would destabilize more, and become more prone to failure, leading to an initial increase in downslope transport. Maybe increased vegetation would stabilize the slope, or if the transportation becomes much more effective, regolith could wash away. Urban dams are susceptible to collapse if they start to leak, creating 'pipes' within the ground and the potential catastrophic collapse of dams.

So when is there faster incision?

Soft bedrock, MODERATE amount of incision, a steeper river slope, and more water discharge.

In order for these flows to occur, the stress tau has to exceed the material strength at a location in the rock, Tau Star. How is tau star calculated and what are its controls?

Tau star=cohesion*friction factor*effective normal stress *Why*: The greater the compressive force, or confining stress, the greater brittle strength. Cohesion is like glue that sticks material together--granite won't just pull apart. The primary control on hillslopes (for material on top of the bedrock) and their cohesion is vegetation, like roots stretching down into the hillslope, or grasses. Some contribution comes from clay minerals, but it is small compared to cohesion, so this is usually what we are talking about. If there is no cohesion in the form of any vegetation, the maximum slope will be about 30 degrees before regolith is sent down, called the angle of repose. But trees add cohesion and add weight/confining pressure and therefore brittle strength so you can have much hillier slopes with overlying regolith material. Friction factor falls between .2 and .6. Higher=greater strength. Very fractured rock or the presence of clay minerals in soils or pieces of rock (creates lubricant bt surface) will give you a friction factor nearer to .2 and hence a greater capacity for landslide. Ex: CA Coastal Range has mountains with very low friction factors where rock types exposed at the surface produce low-friction regolith, either because they're slippery, clay minerals, or fracture-susceptible. On Highway 101 through Sonoma County, there are a ton of evident slumps and stuff. Effective normal stress is determined by weight of overlying material, the normal stress, and reduced by the presence of water which acts against the strength of the material, so N star for effective normal stress is N-P for precipitation/water.

Page 32 skipped, lmfao. Page 33. Let's talk about downslope transport processes on *strong* bedrock slopes, in production-limited landscape.

The Coloumb Relationship still applies, but cohesion is much stronger, and weathering processes may loosen up material near the surface, but downslope transport occurs. 1. Debris flows may be triggered by a section of fractured/broken rock, similar to earlier discussion. I didn't log examples but it's around 50 minutes. 2. Rock fall is what it sounds like. The impact of chunks of rock falling, creating a talus (scree), or little pile near the bottom of the hill. Dislodging may happen bc of regular weathering like ice lenses or just fauna walking. McGee pass in the high Sierra is an example! Taluses might make it hard to walk along these slopes, can be almost 2km high in the Andes in Chile. A cohesion problem. 3. Rock slides are not as common, but are abundant. Fracture happens more deeply than in regolith fractures and creates a tongue-like deposit at the bottom. Denali example, deposits kind of look like volcanoes. Mount Rainier is an example. Deposits a bunch of debris at the glacier beneath. When rock slides dominate downslope transport processes, it's often because the condition of stress>strength was determined by a characteristic slope. That is, at steeper slopes, you get to a slope angle where there is always going to be some failure by rock slide, so it becomes an upper limit on bedrock slopes, hence you see more planar slopes, sometimes corresponding to layers in the bedrock or fracture systems. Half-Dome was made because of a set of vertically oriented fractures causing collapse and leaving that sheer surface. 4. There are rotational slumps. Rotational slumps occur when the failure happens more deeply bc of how the material creates stresses travelling up through the rock/downslope! They occur for bedrock as well.

Water flows aren't strictly /downhill/, so what determines how they flow?

The Piezometric Potential--potential energy determined by difference in elevation from highs of topography and the lows. System of flows and their potential energies determined by elevations. As the piezometric flows store and release potential energy, river is charged by these flows, which is what feeds base flow.

What's the culmination of these facts?

The further you go downhill, the more likely it is that erosion will occur. At the top of the hill, the quantity of water will simply be insufficient to remove material. When erosion starts, there is a very important feedback loop that develops.

What are the factors controlling bedload flux? How is bedload flux related to discharge variability?

The graph is nonlinear--it doesn't move until it moves very fast. If bedload flux is Qsb, kb is a constant, and discharge Qwb^p is water, Qsb=kb*Qwb^p. Imagining a discharge hydrograph with the same volume over a period of time but a different pattern of variation, the hydrograph that doesn't vary very much will tend to never go past the threshold that causes bedload flow, whereas the variable hydrograph won't pass it some of the time by a lower degree, but will cause bedload at certain points. The mean water discharge is the same over time, but the integral of Qsb for each Qw/time graph is extremely different. Climatic/meteorological controls and the other runoff controls, and stuff like feeding aqueducts or dams, will change the river's ability to transport sediment, especially bedload. Sediment can get choked upstream bc of a lack of variability. The Colorado River is an example of sediment accumulation due to a loss of variability.

An aside about velocity and ways to think about erosion

The hillslope acts as a catchmount, meaning thicker layers of water and deeper channels; since velocity is dependent on this depth, velocity will increase downhill, and so this is why erosion rate will increase. We know it's bc of velocity AND because of the slope/depth, we just have to connect those ways with our brains.

What is the transition zone between channels/no channels called?

The morphological threshold of channelization

Why do these forces/stresses increase downhill?

The previous tenent of downhill water flow--that the downhill slope acts as a sort of catch-basin during a runoff event.

What is the law of stream numbers?

The ratio between stream orders--the bifurcation ratio--will tend to be constant. E.g., 9 of order 1, 4 of order 2, 1 of order 1.

More on base flow. What is water table? How is it related to groundwater?

The result of groundwater draining out from the subsurface topographic features like hills or whatever. The 'water table' is the water above the void space filled with water, and above it they're partially filled w water or dry. Water below the water table is groundwater--hills are saturated below this level.

Page 42. Imagine a case where a mountain front with a steep slope meets a canyon with a smoother slope. Why do alluvial fans form in these areas?

The river at the transition zone sees a reduction in slope leading to reduced transport capacity and increased width bc of lack of mountain confinement, and depth of the river channel as a consequence, reducing Qs again. Discussed at 1:11:04. So, as the river goes down, transport capacity goes down, and depositions form, like alluvial fans. River channel tends to build up deposits in one location, but becomes unstable and may break out during flood event, moving back and forth across deposits. Canadian rockies are example. Pictured is a fan by the Ganges. Hindrance to development because of how dynamic the channels are. Alluvial fan in Tuscon being dissected bc of a change in the climate eroding into it. Death Valley has a debris flow, but it's a similar thing. Telescope Peak as well. Hybrid fans, alluvial and debris.

How do these downslope processes interact with rivers at the valley bottom?

The river will receive rock material, and has to deal with that somehow. Might transport the rock as sediment, alluvial sediment. The material might come through channels after water runoff events, into channels via debris flows or the river itself if a debris flow is heavier, creep into drainage networks, sleetwashes forming rills. 24:57, river in Colorado running into sediment after a slide. Will carry them, or maybe get plugged. Boulders may just stay there, or get quarried. In Switzerland, a river plugged by a deposit cut through the material. Sheetwash can occur on pretty gentle slopes especially with sparse vegetation.

Page 22. What is oxidation?

The second important category of chemical weathering. It is any process by which a compound gives up its electrons to another chemical or compound that wants them, overwhelmingly oxygen in geomorphology. Oxygen is electron-greedy and will grab them from another element, particularly iron and aluminum, forming an oxide such as Fe2O3. The joining of the iron with the oxygen starts to tear apart the mineral that the iron was in, and as the oxides forming, we have residual clays, a relatively weaker rock; this is basically equivalent to rusting. In terms of oxidation capacity, analogous to acid weathering capacity, there is eH, the quantity of elements that want to give up electrons to oxidation. Greater than 1 is oxidizing and less than 1 is reducing. Fresh waters are oxidizing while older waters tend to be reducing!

What is the sediment yield?

The sediment yield will have units of mass per time per area of ground surface, and the normalization against area is a way to compare sediment outflows against basin size. Sediment outflow is also directly rated to erosion rate. Sediment yield is like the analogous measurement for sediment transport related to total discharge/runoff Qw. Qs/Ab is formula.

Page 34. As a result of this suite of transport processes, what is the relationship between the strength of the rock mass and the slope angle as the slope erodes down?

The weak layer will be more gently sloped and the stronger area will be more steeply sloped. The weaker area might erode over time, finally reaching a threshold of slope angle that can cause a strong rock to fail at a steep angle. The steep angles kind of remain bc they are stronger. "Creates a steep angle that can fail." 1:04:47. Teton Range in Wyoming has these sedimentary rocks of varying mechanical strength. The Grand Canyon has steep slopes for strong rock and weak slopes for weak rock. A New Zealand study demonstrated this relationship. The granite of Sierras like El Capitan can support stronger slopes.

What are the processes that form river deltas, and where do they occur?

The whole area of the LA mississippi is called the 'delta', which occur where rivers flow into water bodies if they are transporting sediment. Flowing toward ocean=loss of sediment transport capacity=potential buildup of sediment near deltas. A flux of water and sediment from both ocean and river is dependent on the tides--so deltas are dependent on sediment transport from tides, from the oceans (lateral), and from the river. Bird's foot delta forms from river-dominant delta. A wave-dominated delta will have a bending river and straight coastline. Beach deposits. Fluvial channels will be perpendicular to the ocean and create tiny little channels, with the currents moving in and out. Depositing at a high rate, so a tendency for overbank flooding to form 'distributaries'. Fly river in New Guinea is an example. The Ganges. Wave dominated river is Senegal River in West Africa.

How can RGM differ according to drainage density?

There are other geomorphological factors that matter in the hydrograph, like drainage density. In a higher-density watershed, there will be faster discharge during peak storm flow because it is easier for the water to move through channels vs overland flow, the distance is much less. In some landscapes, higher drainage leads to more discharge. But this is true for something like overland flow, not groundwater flow.

All else ceteris paribus, an increase in coefficient C means an increase in incision rate I dot. This being true, how does C change with a change in the presence of sediment flux (mass per time of sediment conveyed by the river channel)?

There will be some incision due to plucking/quarrying if there is no sediment flux, but no abrasion. If we increase sediment flux, abrasion becomes possible, so C goes up. BUT THEN, C dives back down to zero, because the sediment covers the rock so much that the bedrock isn't incised.

What is channel/stream order? What is the Strahler order?

These networks have to fill the spaces of the watershed, within the space after the threshold of channelization that occurs after the ridge crest. This 'order' is a way of categorizing those spaces and designating the location of channels within the network. 'Strahler order' is most common: start at the heads of the channels. The first heads are first order channels. When two orders of the same channel come together, it bumps up their order. Shows example 46. If you have two channels of diff orders coming together, the order doesn't increase. You will count channels of a certain order. Used to define its position in the drainage network, which shows certain characteristics of channels in their position--low order channels will be drier and choked by sediment, and high order channels will have high discharge, etc. Used a lot in land management!

SECTION 4 PAGE 5--Rivers, aka big ass channels, can slice their way down into the bedrock. What is this called and why is it important?

This is called river incision, which creates the slopes of the hills--they can't erode below the rivers that sliced them. River incision creates the pace of erosion for hillsides. 1:03:48 actually made this make more sense.

What causes a river to braid and not meander? When does it cross that threshold?

This threshold is slope, though it varies with discharge. But at any given Qw, higher slopes mean braided.

Steppin' back for a minute to evaluate the form alluvial rivers take, braided rivers, very large sediment supply compared to transport capacity.

To deal with sediment, the river deposits the sediment within its channel, little islands of sediment forming 'braids'. The actual river channel conveying the water consists not of a single channel but many threads, the sediment dealt with in a piecemeal fashion. The braided rivers are a function of bar instability, concentrating sediment causing water to be funneled into more narrow channels with the faster flow to accommodate water, preventing further deposition of the sides. Example is South Island of New Zealand near mount cook. Africa.

Runoff rate in the USA (study and answer questions about these maps)

Topograhic effect (trapped winds, Sierra and Western US), ocean temperature effect (near Gulf of Mexico, but not near cold CA oceans)

The fundamental thing to know about hillslope processes:

Transport happens by water carving and by mass-wastage processes like landslides. The shear stress on a hill is at the start of section 4, weight of overlying material*gravity*depth*sin of the hill angle. Thus, for potential failure planes, you have to have a stress that is greater than the material strength of the rock. For failure: Tau > tau star (mechanical strength of rock) We know steeper slopes mean more transport, that Tau number for the shear stress downslope. This is the normal situation. Special cases: at something like a ridge crest, tensile stresses will form at the crest bc of the pulling-apart shear stresses, creating mini normal faulting. In the case of something like El Capitan, that are vertical or sheer, the shear stresses are oriented toward one another creating compression and have nothing to prevent accompanying horizontal forces, creating great fractures. If undercut by a river, the angle is not just the slope, but also the angle moving down to the river; there is less material strength. If the stress exceeds the material strength it will be like the rock is brittle and a mass wastage event will result.

Page 16 of section 4. What is bank-full or bankfull discharge, and what effects does overbank flooding have on transport of water?

Typically, at an average moment in time, the water is contained within the river channel. What happens as discharge increases during a stormflow event-->the water level will rise. Water level even with landscape is bank-full discharge; a further increase would cause overbank flow. Overbank water flows very slowly bc there is slow depth and a lot of coverage on the land generally, hence it is temporarily 'stored'. This reduces the discharge downstream compared to what it would be otherwise. Once the stormflow or increased discharge event is over, the water level will fall and the overbank water will return. Thus, the discharge hydrograph downstream is greatly affected by the ability of a river to overbank flood. The hydrograph will be much flatter as downstream flow is delayed and reduced. Levees may be constructed to prevent overbank flooding. They confine the water in the channel, and eliminates the effect of temporary storage, so downstream, the discharge hydrograph is much peakier. The levee solves the flooding problem locally, but amplifies it downstream!

If the hill is steep, where do we expect to find the threshold of channelization? (BOTTOM OF SEC 4 PAGE 4)

Up the hill, bc it takes less time for the water's force to exert sufficient power (is there a real physics term for this?? lol) to cause erosion

What is cohesion?

Vegetation or other forces that hold together the soil or regolith and reduce the impact of these forces. Illustrates more realistic concept of cohesion at 33:27.

Page 39, bedload transport.

Very important for the morphology of the river channel. Consider a water column of a river again. If we measure the rate of water flow right next to these particles, we'll find there's a ton of variations in velocity right next to the bed from second to second, with rough eddies exploding then evening out bc they have to be right next to the bed. Bedload movement happens in big peaks of velocity when these eddies explode. Bedload transport flux as a mean of stress on the bed over time, Tau B, or as discharge, the bedload won't move until a critical value Tau C causes movement. This Tau C is a constant .6*(density of rock-density of water)*particle distribution. Denser rocks are harder to move, causing stuff like the Placer slope deposits.

Page 14 of section 4. At a specific location in a river channel, imagine you want to know how flow varies with depth in the channel, and what ultimately determines the average velocity in the channel.

Water is slowed down by dragging against the material on the sides of the channel, so the flow is fastest in the center and near the surface (versus near the bed). Importantly, there is also a zone of rapid shearing near the bed, whereas the top 80% of the center moves mostly at the same rate. The flow is roughly the same on average, but fluctuates hugely second to second; at any second in time, the eddies of water colliding cause volatile changes in velocity.

What determines Qw/what is it related to?

We learned that Qw=(basin area*avg runoff rate)-changes in storage. The changes in storage actually become incredibly important. The rises in the hydrograph are related to runoff events--precipitation and snowmelt. Usually split into two parts, called /storm flow/ and /base flow/ (continuing discharge in a river separated in time from any snow melt/storm event; a lag time after water is delivered to the landscape).

Coloumb Friction Factor, aka Tau Star--how is it determined? What are the factors controlling normal and effective stress? What is the role of water in hollows on hillslopes?

We talked last time about the controls affecting friction factor and cohesion. Now let's discuss the effective normal stress, the third component. Frictional stress is created by rough material, and when rough materials are put under pressure/compressed, rock becomes more brittle-ly strong. Normal compressive force makes the rock stronger. But water may flow among the rocks and acts against the strengthening effect, so the water pressure is called the effective stress. There is an association between mass wastage events and rainstorms/lots of wet weather. Association between saturation of the ground and these slides. Millbrae, Indonesia. Weakening by deforestation in Indonesia has reduced the cohesion factor and the normal stress, so monsoons push it to failure because of the effective stress introduced. A perched water table more form above the bedrock after regolith saturation. A little water means not that much water pressure, but the effective stress grows as storm conditions grow, the water table thickens, and failure might occur. As water builds up over this material, the rate of the water increases, increasing downslope stress with overlying density, but a normal stress responds with a strengthening in slope. The weight of the water isn't in the equation bc it is evened out by its compressive strength accompanying it. *Hollows*: The water saturating will tend to accumulate within little 'pits', the places below steeper ridges where the water accumulates and pressure grows! Failure tends to occur at hollows instead of steeper ridges.

What is chelation?

When organic compounds (thingies haha) in the rock are able to grab at heavy metals essentially act as chemicals able to affect the solubility of the rock compounds it is occupying. Usually include sugars and amino acids!

When is a river not incising?

When the bedrock is protected by layers of sediment.

What is a runoff generation mechanism?

When water is delivered to the surface, it makes its way to a nearby channel. The path or paths that the water takes to this channel are the RGM. It can deliver the water fast, and in that case there will be a large spike during storm flow, or it can deliver more slowly, giving us a more flat hydrograph.

Where do you find soils at each stage of this progression?

Where each stage occurs is determined by: 1. the rate and duration of weathering processes. 2. the removal via transport. If there is a hillslope getting weathered, the removal of material to expose bedrock is influenced by how fast it is moved away and the rate+duration. Removal by downslope transport effectively limits the duration of weathering processes by exposing more un-weathered rock, resetting the clock. This is also combined with physical weathering!

What is the interfacial pressure layer, and how is it related to why water is drawn to ice lenses? Salts may also do a similar thing to ice lenses btw.

Where the rock meets the ice (the interface) there will be a low-pressure layer of liquid water because the rock and the water will repel one another. That is the interfacial pressure layer. The ordinary pressure of the water beneath it will flow toward this low pressure layer.

Is there a relationship between the length of a drainage basin and its area? (END OF SECTION FOUR PG SEVEN)

Yes--length squared is approximately proportional to drainage area. There is a tendency for a drainage network to expand as it grows out. The deviations tend to be interesting--it indicates something about Ks, tectonism deforming the size of the basin (as in the Himalaya).

What are the factors of bodies of water that may cause more chemical weathering to rock? Why does most rock weathering occur near the surface/the interface of water and the surrounding elements? What are some exceptions?

Why are these weathering processes more powerful in fresh water?-->There are more of these chemicals floating around. Rock that weathers easily will be acid and oxidizing, a low pH and a high eH. He makes a 4-square quadrant of these types of bodies of water. Where does water exist on this PCM? Acid and oxidizing: Rain, bc of the presence of co2 from decomp and oxygen. As it's fresh, it remains acidic, but the oxygen capacity gets used up. Acid and reducing: Shallow water that hangs around! Alkaline and reducing: Alkaline and reducing: Groundwater--meaning that the rock is not necessarily weathered down here and can maintain its strong mechanical form. Volcanic environments can be an exception bc of the chemicals floating around.

Threshold does not correspond perf to what happens irl

Will be effected by upward processes

What was the 'nightmare scenario' here?

Wind flowing east imposed a shear stress on the current creating a 'storm surge', raise lake Ponchitrane, flood the city, erode the levies, and flood it from the other side. This is basically what happened with Hurricane Katrina, but not exactly. Water levels rose, pushed on the slippery, failure-prone material, and the levies broke at certain locations bc of the non-uniform sediments.

Something very important for the exam is .... rhyolite..... codeword numbah one. Lol I mf hope it's number one

check 16 and 26 since homie decided to be james bond

Himalaya belt

example of a topographic effect

Consequences of runoff (PG THREE SECTION 4)

filler--19:00 lecture 14

james bond 2

isostasy

revisit section abt base flow, it's been about 40 minutes

realized i can declare my double major so i stopped paying attn lol

Recap of the times in history when the bottom part of the Mississippi basin acts like an alluvial fan does, overcoming its own barriers and rerouting its channels. Used to stretch down through Louisiana. Pics on Raisz map:

southern MI and LA

Page 31. What is soil creep? What are its two broad types?

the slow downward progression of rock and soil down a low grade slope. The potential for creep is dependent on slope. 1. *Deformational creep*. A shear stress on the surface might create a stress profile within the entire regolith, making it act as a ductile material, potentially deforming nearby vegetation/houses. If there is strata in the regolith, it might get tilted or bent. 2. *Disturbance creep* may arise from particle getting jostled and shuffled around, gravity pulling it downslope. Something like burrowing rodents. Or, a tree falling over if it has roots in the subsurface situated in unconsolidated material will make a hollow bc it wanted to cling to the material; over time, as rain happens, the stuff will get washed down and a preference will be for it to land downslope. Big fauna might also push down on the soil with its weight, slowly moving it downhill due to net tendency, like cattle, creating 'terrace sets' aka paths for cows. Rain splash too, but not significant without vegetation. The more frequent disturbances are and the larger they are, the more you know it happens. Deformation is sort of dependent on the ductile strength of the soil. Examples shown: the tilting of strata and fence posts in deformational creep, especially its non-uniform properties.