Dynamic earth Test 3

Some processes that over steepen slopes include :

-stream erosion -wave erosion -volcanic activity -tectonic uplift -human construction

how is discharge related to velocity?

: Discharge is the amount of water flowing pass a point on a stream per unit time. The stream velocity and discharge increases along the outside of the meander, a stream will begin to pick up more sediment, by eroding the bank and thus deepening the channel there. o Velocity varies vertically & laterally across the river o Discharge is dependent upon the velocity of water & size of channel at a given point

What factors affect the velocity of the streams?

: The velocity of streams depend on stream gradient (slope of surface over which it flows), the size and shape of the channel, channel roughness, and discharge. Size and shape of the channel affect a streams velocity in different ways. A straight stream having a channel with a symmetrical, U-shaped cross-section, the fastest current would be in the center of the stream, slightly below the water surface (close to the bed and channel walls, velocity is slowed by drag effects). Streams are never perfectly straight, nor symmetrical in cross-section, so within streams are currents of different velocities.

Precipitation - evaporation - infiltration - runoff:

: are all a part of the Hydrologic cycle • As rain sinks into the ground, it becomes groundwater, the mass of water stored beneath the Earth's surface. Each of the environments in which water is stored is a reservoir. The oceans, glaciers, polar ice, underground waters, lakes, rivers, the atmosphere and even the biosphere are considered natural reservoirs of water on the earth.

What is meant by "rejuvenation", why does it occur, and what are the characteristics of rejuvenated streams and their valleys?

A Rejuvenated stream is one in which it has a V-shaped valley and no flood plain (characteristics of stream far above base level). However, it also possesses a complex and broad meander belt (a characteristic of stream close to base level). These features are evidence that the stream was once close to base level (when it developed the meanders across a broad flood plain). Now the stream is far above base level and therefore spending its energy down cutting into the rock. Characteristics of rejuvenated streams include: • entrenched meanders • paired river terraces • terraces

Thoroughly understand the concept of a graded stream and how they behave and respond to change:

A graded stream is a system in equilibrium, one in which there exist a balance of the erosional and depositional processes. It is a stream that is transporting just that amount of sediment it is capable of, for its velocity and discharge. While a graded stream might be both picking up new sediment by erosion and depositing sediment, there is no net erosion or net deposition. While it might be argued that a graded stream is a concept, and that a truly graded stream does not exist, it is a concept that is certainly approached in nature, and that one that is powerful in predicting stream behavior. Indeed accepting the premise that all streams are either graded or are striving toward that condition, will provide you with a way anticipate results of external stimuli on a stream.

levee

A levee dike (or dyke), embankment, flood bank or stop bank is an elongated naturally occurring ridge or artificially constructed fill or wall, which regulates water levels. It is usually earthen and often parallel to the course of a river in its floodplain or along low-lying coastlines.

meander belt

A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley.

drawdown

A reduction in the volume of water in a lake or reservoir; The act of reduction or depletion; The result of reduction or depletion; A change in hydraulic head in a well or other body of water

How does the longitudinal profile of a stream evolve through time?

As a stream deepens its valley, by downward erosion, over steepened valley walls undergo mass wasting, thereby feeding sediment to the stream to be carried away. Valley walls thus retreat away from the stream, i.e. valleys widen and highland dividends between valleys become lower and less extensive. Widened valley floors provide larger areas over which meanders migrate and flood plains expand. So as streams evolve in their long profile - toward base level, so too does the landscape across which streams flow. As streams evolve toward base level (or along reaches of streams where the long profile begins to flatten) sediment supplied by erosion of valley walls supply a larger component of the stream load.

How does current velocity vary within stream channels, and along its length?

As stated above in the aforementioned, streams are never perfectly straight, nor symmetrical in cross-section, so within streams are currents of different velocities. o A stream's velocity depends on position in the stream channel

How can you recognize evidence of past mass wasting events?

Because mass wasting events are one of the primary agents of landscape evolution, it is not surprising that we can see evidence of previous events recorded in the landscape. Hummocky topography is the name given to any irregular land surface consisting of shallow undrained depressions and low hills. This landscape is the primary characteristic of past landslides occurring in the area. Tilted telephone poles or fence posts can indicate that the surface has undergone slow creep. Slumps are generally identified by the steep scarps that develop along their uphill boundary. Large piles of talus at the base of cliffs are good evidence for rock slides or rock falls.

What (if anything) can be done to lessen the possibility of future such events in areas prone to mass wasting?

Construction of retaining walls, putting in drains through the walls, reducing the angle of slope through construction of terraces and planting grasses or other plants whose roots anchor the slope are sometimes effective means of slowing downslope movement. Builders frequently sink pilings through unstable debris down to firm bedrock. Rock slides and rock falls are sometimes prevented by removing sliding rock layers or inserting bolts through the unstable rocks. In some locations where water is increasing the possibility of slides, wells are drilled into the water table to pump out the water and drains are inserted into the cliff faces.

ground water problems that face some areas of the country and world; problems such as depletion, contamination, saltwater intrusion, subsidence (and land rising)

In some places, dissolution may thin the roof of a limestone cave so much that it collapses suddenly, producing a sinkhole, a small, steep depression in the land surface above the cavernous limestone formation.

All the thing that influence mass wasting include :

Nature of materials involved (rocks, sediments, soil, ect.) steepness of slope water vegetation heights climate (such as freeze-thaw cycles) presence and orientation of planes of weakness (such as joints, bedding, foliations) some human activities (think of examples

How does the longitudinal profile of a stream vary along its length?

Near the headwaters (source) of a stream its gradient is the steepest. With increasing distance downstream the gradient becomes less. Much of the length of this stream is high above base level. Where the stream is close to base level (near its mouth) its gradient becomes vanishingly low, thus it has little ability to deepen its channel by downward erosion.

artesian

Relating to or denoting a well bored perpendicularly into water-bearing strata lying at an angle, so that natural pressure produces a constant supply of water with little or no pumping

meander

Streams develop curves or bends called meanders. Water flowing into a meander tends to "pile up" on the outside of the meader, and the stream velocity is higher there on the outside of the bend, than it is on the inside.

paired river terraces

Terraces of the same elevation on opposite sides of either a stream or river are called paired terraces. They occur when it down cuts evenly on both sides and terraces on one side of the river correspond in height with those on the other side. Paired terraces are caused by river rejuvenation.

What determines the competence and capacity of a stream?

The competency of a stream is dependent upon its velocity, and the capacity of a stream id dependent upon the streams discharge (which is the amount of water flowing pass a point on a stream per unit time)

permeability

The ease with which fluid is transmitted through a rock's pore space is called permeability.

zone of saturation

The phreatic zone, or zone of saturation, is the area in an aquifer, below the water table, in which relatively all pores and fractures are saturated with water. The phreatic zone may fluctuate with changes of season and during wet and dry periods.

recharge

The replenishment of an aquifer by the absorption of water

lag-time

The time between initial inundation and onset of rapid sediment accumulation

What factors are important in determining whether rain will infiltrate and become part of the ground water, or runoff as sheet or channel flow?

Whether or not it gets soaked up in plants and returned to the atmosphere via transpiration. The rainwater may also evaporate directly back into the atmosphere.

ox-bow lake

a U-shaped body of water formed when a wide meander from the main stem of a river is cut off to create a lake. This landform is called an oxbow lake for the distinctive curved shape, named after part of a yoke for oxen.

point bar

a depositional feature of streams. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on the inside of a stream bend, being very similar to, though often smaller than towheads, or river islands. Because stream velocity on the inside of a meander slows down, this loss of energy results in the stream depositing some of the sediment it is carrying. This deposition begins to fill in the channel along the inside of the curve forming point bars (generally sand or gravel)

pediment

a gently inclined erosional surface carved into bedrock. It is thinly covered with fluvial gravel that has developed at the foot of mountains. It develops when running water erodes most of the mass of the mountain. It is typically a concave surface gently sloping away from mountainous desert areas. Lateral plantation or erosion by streams are responsible for carving pediments.

delta

a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river. Over long periods of time, this deposition builds the characteristic geographic pattern of a river delta.

avalanche

a mass of material moving rapidly down a slope. It is typically triggered when material on a slope breaks loose from its surroundings; this material then quickly collects and carries additional material down the slope. There are various kinds include rock avalanches (which consist of large segments of shattered rock), ice avalanches (which typically occur in the vicinity of a glacier), and debris avalanches (which contain a variety of unconsolidated materials, such as loose stones and soil).

Hydraulic head

a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance (or bottom) of a piezometer

channel

a stream may be referred to as this

terrace

a terrace is a step-like landform. A terrace consists of a flat or gently sloping geomorphic surface, called a tread that is typically bounded one side by a steeper ascending slope, which called a "riser" or "scarp." The tread and the steeper descending slope (riser or scarp) together constitute the terrace. Terraces can also consist of a tread bounded on all sides by a descending riser or scarp. A narrow terrace is often called a bench.

downstream, a river generally has...

a wider, deeper, smoother channel, and a greater discharge. It is carrying the waters supplied by tributaries. It is true, however, that along a given segment or reach of a stream, if its gradient is increased (such as by construction of a meander cutoff of comparable channel size), the velocity will be increased - despite the fact that it's a temporary condition, because equilibrium will be restored eventually.

floodplain

also called Alluvial Plain, flat land area adjacent to a stream, composed of unconsolidated sedimentary deposits (alluvium) and subject to periodic inundation by the stream. Floodplains are produced by lateral movement of a stream and by overbank deposition; therefore they are absent where down cutting is dominant. Any erosional widening of one bank is approximately equaled by deposition on the opposite side of the channel in the form of bar development along the inside of meander bends. Thus, the simplest floodplain is made up of a strip of sinuous scrolls immediately adjacent to the stream.

cut-bank

also known as a river cliff, is an erosional feature of streams. Cut banks are found in abundance along mature or meandering streams, they are located on the outside of a stream bend, known as a meander. They are shaped much like a small cliff, and are formed by the erosion of soil as the stream collides with the river bank. As opposed to a point bar which is an area of deposition, a cut bank is an area of erosion.

incised or entrenched meanders

an entrenched river is a river that is confined to a canyon or gorge, usually with a relatively narrow width and little or no flood plain, and often with meanders worn into the landscape. Such rivers form when the base level of erosion is rapidly lowered, so that the river begins down cutting into its channel faster than it can change course (which rivers normally do on a constant basis).

sinkholes

contribute to a distinctive form of topography known as karst topography.

valley

elongate depression of the Earth's surface. Valleys are most commonly drained by rivers and may occur in a relatively flat plain or between ranges of hills or mountains.

Streams

erode the surface of the earth (picking up sediments), they transport sediments, and they deposit sediments

What landforms are produced by streams?

erosional and depositional

peneplain

gently undulating, almost featureless plain that, in principle, would be produced by fluvial erosion that would, in the course of geologic time, reduce the land almost to base level (sea level), leaving so little gradient that essentially no more erosion could occur.

What determines the velocity of a stream?

gradient, discharge, channel characteristics

alluvial fan

is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fan.

headward erosion

is a fluvial process of erosion that lengthens a stream, a valley or a gully at its head and also enlarges its drainage basin. The stream erodes away at the rock and soil at its headwaters in the opposite direction that it flows. Once a stream has begun to cut back, the erosion is sped up by the steep gradient the water is flowing down. As water erodes a path from its headwaters to its mouth at a standing body of water, it tries to cut an ever-shallower path. This leads to increased erosion at the steepest parts, which is headward erosion. If headward erosion continues long enough, it can cause a stream to break through into a neighboring watershed and capture drainage that previously flowed to another stream.

landslide

is a general term referring to all slides, flows (even falls) that occur at a moderately fast rate.

Hydraulic gradient

is a vector gradient between two or more hydraulic head measurements over the length of the flow path. It is also called the Darcy slope, since it determines the quantity of a Darcy flux, or discharge.

aquifer

is a wet underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, or silt) from which groundwater can be usefully extracted using water well.

karst topography

is characterized by sinkholes, caverns, and a lack of surface streams. Underground drainage channels replace the normal surface drainage system of small and large rivers. Short, scarce streams often end in sinkholes, detouring underground and sometimes reappearing miles away. Karst is most strongly advanced in regions with three characteristics: o High-rainfall climate, with abundant vegetation (providing carbon dioxide-rich waters) o Extensively jointed limestone formations o Appreciable hydraulic gradients

How will the stream adjust in order to restore a graded condition?

it will deposit its sediments

what is meant by "base level"?

its the level below which a stream cannot erode.

Things that reduce the infiltration capacity of the ground results in...

more runoff and greater flooding potential

Karst features such as sinkholes and caverns, hydrothermal waters

o Hydrothermal waters, hot waters deep in the crust, migrate rapidly upward without losing much heat and emerge at the surface, sometimes at boiling temperatures. Hydrothermal waters are loaded with chemical substances dissolved from rocks at high temperatures. As long as the water remains hot, the dissolved material can remain in solution, but as the waters quickly cool at the surface, they may precipitate various minerals. o Most of the hydrothermal waters of the continents derive from surface waters that percolated downward to deeper regions of the crust. These surface waters originate primarily as meteoric waters such as rain, snow, or other forms of water from the atmosphere. The other source of hydrothermal waters is water that escapes from magma.

What factors influence a streams capacity to transport sediment (and pick them up by erosion) and its deposition of sediment?

o Steam velocity o Depending on the competency of a stream, sediment is carried either in suspension in the water column, or the sediment is dragged, rolls, or hops along the bottom (bed) of the stream channel. o If velocity slows down, sediments then begin to deposit themselves

creep

occurs at the slowest rate of mass wasting processes, often at a cm/year or less. The creep of soil and other unconsolidated material can be a "grain by grain" process facilitated by freeze-thaw cycles. As water between grains crystallizes to ice and expands, a grain is pushed away from the surface of the ground. On thawing, that grain falls under the influence of gravity, ever so slightly farther down slope than it was. Worms, insects, small rodents, and plants move grains of soil around.

cone of depression

occurs in an aquifer when groundwater is pumped from a well. In an unconfined (water table) aquifer, this is an actual depression of the water levels. In confined (artesian) aquifers, the cone of depression is a reduction in the pressure head surrounding the pumped well.

Stream velocity...

on the inside of a meander slows down

braided stream

one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars. Braided streams occur in rivers with high slope and/or large sediment load. Braided channels are also typical of environments that dramatically decrease channel depth, and consequently channel velocity, such as river deltas, alluvial fans and peneplains.

porosity

or void fraction is a measure of the void (i.e., "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0-1, or as a percentage between 0-100%.

cut-off (or chute)

shortcut across a meander. loop that shortens and straightens the course of the stream. Chutes are formed by lateral erosion of the bank of the upstream arm of a loop, which causes the stream to cut through the neck of the loop into the downstream arm.

Meandors

streams develop these curves or bends. Water flowing into it tends to pile up on the outside of it, and and the stream velocity is higher there on the outside of the bend, than it is on the inside.

Understand Darcy's law - i.e. what controls the velocity of ground water flow, and understand factors that affect the rate of recharge of an aquifer.

tells us that water flows, at rates proportional to the slopes of the aquifers, between their recharge areas and the areas of discharge from wells. In the western plains, the slopes are gentle and waters move slowly through the aquifers, recharging them at low rates. At first, many of these wells were artesian and water flowed freely. As more wells were drilled, the water levels dropped, and the water had to be pumped to the surface. As extensive pumping withdraws water from some aquifers faster than the slow recharge from far away can fill them, the reservoirs are being depleted. Darcy reasoned that the relationship between flow and hydraulic gradient should hold whether the water is moving through a porous sandstone aquifer or an open pipe. The water would move more quickly through the pipe than the pore spaces in an aquifer, and thus Darcy included a measure of permeability in his final equation, so that, other things being equal, the greater the permeability and thus the greater the ease of flow, the faster the flow. Darcy's Law can be expressed as the velocity (V) is proportional to the vertical drop (h) divided by the flow distance (l). K is the hydraulic conductivity (a measure of permeability). K also depends the properties of the fluid, especially density and viscosity. The ratio h/l is referred to as the hydraulic gradient; h is the hydraulic head, it is a pressure. V = K (h/l)

precipitation

that does not evaporate either infiltrates the ground or runs off the surface.

Mass wasting

the down slope (hill) movement of rock, soil, or sediment under the influence of gravity;events occur in different ways and at different rates, depending on the types of materials involved (such as rock, soil or earth, mud, debris) and the motion involved (fall, slide, flow).

discharge

to allow (a liquid, gas, or other substance) to flow out from where it has been confined

water table

underground surface below which the ground is wholly saturated with water; "spring rains had raised the water table"

slump

when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope. Movement is characterized by sliding along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope. The speed of slump varies widely, ranging from meters per second, to meters per year. Sudden slumps usually occur after earthquakes or heavy continuing rains, and can stabilize within a few hours. Most slumps develop over comparatively longer periods, taking months or years to reach stability. Note that the ground surfaces on the intermediate blocks dip slightly toward the master sole of the slump. The distal end (toe) of the slump moves along a less steep surface. Often the toe of slumps moves more as flows than as slides.

aquiclude

which is a solid, impermeable area underlying or overlying an aquifer. If the impermeable area overlies the aquifer pressure could cause it to become a confined aquifer.

What factors affect the infiltration capacity of the ground?

•the permeability of the surficial material •the slope of the land •amount and type of vegetation •recent amounts of precipitation (degree of saturation of the land) •Runoff is either in the form of sheet flow (such as during floods) or it is confined to channels. Channelized runoff is stream flow.