Chapter 14: Running Water - Geology of Streams and Floods Reading Notes

alluvium-filled valley

- rise in base level or decrease in discharge will create this - stream's base level rises, its discharge decreases - or its sediment load increases, the valley floor fills with sediment, creating this - surface of this becomes broad floodplain

meanders

- river channel winds back and forth - series of snake-like curves - winding back and forth in snake-like curves

Dendritic

- rivers flow over fairly uniform substrate with fairly uniform initial slope - LOOKS like pattern of branches connecting to trunk of decidous tree

stream channels

- running water collects here - form by downcutting and lengthen by headward erosion - carry water from sheetwash, lakes, springs, and melting snow or ice - lower than surrounding area and gravity - since its lower than surrounding ground, sheetwash in adjacent areas start to head toward it

stream-gauging station

- stream discharge can be determined by this instrument - measures velocity and depth of the water at several points across the stream - measure cross sectional area, depth, and average velocity of stream - velocity slower near banks

braided stream

- stream divides into numerous strands weaving back and forth between elgonate bars of gravel and sand - during normal flow, the sediment settles out and chokes the channel - name emphasizes streams entwine like strands of hair in a braid - if choked with sediment, may split into numerous enterwined channels separated from one another by gravel bars

headwaters

- stream starts from source - some located in mountains collecting water from rainfall or from melting ice and snow

meandering streams

- streams that develop many meanders - course - landforms include

flash floods

- sudden and short lived - floodwaters rise so fast that it may be impossible to escape from path of water - occur during unusally intense rainfall, result of dam collapse, or dam collapse - canyon or valley may fill to level many meters above normal - can be unexpected in arid or semiarid climates

ephemeral streams (water table below)

- temperature climate lies below water table - springs add water from _______, so stream contains water even between rains

stream gradient

slope of the stream channel

valley

deep trough creates a landform that slopes gentle

drainage basin

drainage network collects water from a broad region

whitewater

mixture of bubbles and water

when floods happen

(1) during abrupt, heavy rains: water falls on the ground faster than it can infiltrate and thus becomes surface runoff (2) after a long period of continuous rain: ground becomes saturated with water and can hold no more (3) heavy snows from previous winter melt rapidly in response to sudden hot spell (4) dam holding back lake or reservoir, levee, or retaining wall holding back river/canal collapses and releases water that it held back

discharge formula

- CALCULATE: D = Ac X va - Ac = area of the stream, as measured in an imaginary plane perpendicular to the stream flow - va = average velocity at which water moves in the downstream direction

competence

- ability to carry sediment - depends on flow velocity - maximum particle size it carries - high = can carry large particles - low competence = can carry only small particles - depends on water velocity - fast, turbulent moving = greater competance, carry bigger particles (ie. flood) - where velocity decreases, sediment settles out

natural levees

- accumulation of sediment - creates pair of low ridges - may grow large that floor of channel may become higher than surface of floodplain - build up between the channel and floodplain from sediment dropped as flooding river starts to pill out of its channel

pattern of stream flow

- altered over time on continental scale - ie. South America and Africa were adjacet to each other in Pangea

stream discharge

- amount of water passing through cross sectional of stream in given time - during the spring may be double or triple the amount during a dry summer - flood may increase the discharge to more than a hundred times normal - depends on factors as.... ^ watershed area ^ climate

distributaries

- area of green agricultural lands broadens into a triangular patch - on the delta, trunk stream divides into smaller channels

drainage network (in context with river systems)

- array of connected streams - consisting of trunk stream into which numerous tributaries flow

dam construction

- both positive and negative results - POSITIVE: provide irrigation water and hydroelectric power, trap floodwaters and create popular recreation areas - NEGATIVE: destroys whitewater streams of mountainous areas, alters ecosystem of drainage network by forming barriers to migrating fish, decreasing nutrient supply to organisms downstream, removing source of sediment for delta, eliminating seasonal floods that replenish nutrients in landscape

floodplain

- broad flat area bordering the stream - During floods, a stream may overtop the banks of its channel and spread out over this - most meandering stream channels cover relatively small portion of broad, gently sloping - water overtops edge of stream channel and spreads out over floodplain

artificial levees

- built concrete flood walls to increase channel's volume - made of sand and mud - isolate discrete area of floodplain

average velocity of stream water (va)

- can be difficult to calculate because the water doesn't all travel at the same velocity - water near the channel walls or streambed (the floor of the stream) moves more slowly than water in the middle of the flow - fastest-moving part of the stream lies near the surface in the center of the channel - where this decreases, sediment settles out 1. friction along the sides floor of the stream slows the flow 2. turbulence

pollution

- capacity of rivers to carry this has been exceeded - treamforming them into deadly cesspolls - some directly poison aquatic life - include raw sewage, storm drainage from urban areas, spilled oil, toxic chemicals from industrial sites, floating garbage, excess fertilizer, and animal waste

watershed

- catchment - feeds it into the trunk stream - carries the water away

effects of urbanization

- changes both the volume of water reaching the stream and the length of the time lag - developers transform fields and forests into parking lots, roads, and buildings - layer of impermeable concrete and asphalt prevents rainfall from infiltrating

Abrasion

- clean water has little erosive effect, but sediment laden water acts like sandpaper - grinds or rasps away at channel floors and walls - places where turbulence produces long-lived whirlpools,

drainage network

- collects water from broad drainage basin or watershed via numerous tributaries - carry water to trunk stream and eventually to standing body of water - array of interconnecting streams together constitute this - carry water from watershed to sea - reach into all corners of a region - provides conduits for the removal of runoff - variety of different geometries - often form with streams

broad floodplain

- covered with water only during floods - may develop on either side of the stream

canyon

- creates slot with steep wall - deep trough creates a landform that slopes steeply - places where stream downcuts through its substrate faster than walls of stream collapse - typically form in hard rock which can hold up steep cliffs for a long time

point bar

- cresent shaped, wedge shaped deposit - bordering shoreline of inner curve develops - stream channel makes a broad curve - water slows along the inner edge of a curve - inside edge = water slows down so competence decreases and sediment accumulates, forming this

longitudinal profile

- cross-sectional image showing variation in the river's elevation along its length - elevation change its length is concave up - roughly a concave-up curve - illustrates stream's gradient is steeper near its head- waters (source) than near its mouth - tend to concave upwards

outer arc

- current flows faster here - causing erosion

inner arc

- current flows slower - where it drops sediment

antecedent streams

- cut while the land beneath them uplifts - existed before range uplifted - tectonic activity (subduction or collision) cause mountain range to rise beneath already established stream - if stream downcuts as fast as range rises, it can maintain its course and will cut right across the range - range rises faster than stream downcuts - new highlands change stream's course so flows parallel to range face

slot canyon

- downcutting by stream happens faster than mass wasting on walls - canyon widens as stream undercuts walls

dry wash (arroyo or wadi)

- dry bed of an ephemeral stream - streams whose watersheds lie entirely within arid region tend to be this way

delta

- eventually river reaches standing body of water and slows down - sediment gets deposited to form this - trunk stream divides into distributaries

ephemeral streams

- flow only for part of the year - some flow only for tens of minutes to a few hours, following a heavy rain - Streams that do not have a sufficient upstream source, and whose beds lie above the water table - water that fills a channel due to a heavy rain or a spring thaw eventually sinks into the ground and/or evaporates, and the stream dries up - streams whose watersheds lie entirely within arid region tend to be this way

ridge

- highland - separates one watershed from another

undermining

- occurs when rising water levels increase the water pressure on the river side of the levee - forcing water through sand under the levee

"water gap"

stream- carved notch through a ridge

sea level

ultimate base level for drainage networks

headward erosion

- As its flow increases, a stream channel begins to lengthen at its origin - process that occurs for 2 reasons 1. when the surface flow converging at the entrance to a chan- nel has sufficient erosive power to downcut 2. locations where groundwater seeps out of the ground and enters the entrance to the stream channel

delta plain

- Distributaries can provide sediment that fills the resulting space - delta's surface remains at or just above sea level - forming a broad, flat area

Radial

- Drainage networks forming on the surface of a cone shaped mountain flow outward from the mountain peak - LOOKS like spokes on a wheel

Trellis

- In places where a drainage network develops across a landscape of parallel valleys and ridges - major tributaries flow down a valley and join a trunk stream that cuts across the ridges - LOOKS like map pattern resembles a garden trellis

Rectangular

- In places where a rectangular grid of fractures (vertical joints) breaks up the ground, channels form along the preexisting fractures, - streams join each other at right angles - looks very geometric

dissolved load

- Running water dissolves soluble minerals in the sediment or rock that it flows over - groundwater seeping into a stream brings dissolved minerals with it - ions of these dissolved minerals constitute this

stream

- any body of running water that flows along a channel - drain water from the landscape and carry it into lakes or to the sea - erode the landscape and transport sediment and debris to sites of deposition - generally stays within the confines of its channel - can receive input of water from all of these reservoirs - erode by scouring, breaking and lifting, abrasion, and dissolution - carry sediment as dissolved, suspended, or bed loads - steeper regional gradients toward their sources - gentler gradients near their mouths - two types: braided, meandering

superposed streams

- attain their shape before cutting into rock structure - establishes geometry while flowing over uniform substrate above an uncomformity - preexisting geometry has been laid down on underlying rock structure - streams flow over horizontal beds of strata that uncomfortably overlie folded strata - streams eventually erode down uncomformity and start to downcut into folded strata - maintain earlier course - ignore structure of folded strata

recurrence interval

- average time between successive geologicial events - flood of given size is defined as average number of years between successive floods of at least this size - flood along a particular river reflects the size of a flood - ie. flood

pattern of tributaries and trunk streams

- defines map pattern of drainage network - depends on the shape of the landscape and the composition of the substrate

factors affecting valleys and canyons

- depends on the rate at which downcutting occurs relative to the rate at which mass wasting causes the walls on either side of the stream to collapse

efficiency of erosion

- depends on the velocity and volume of water and on its sediment content. - large volume of fast-moving, turbulent, sandy water = more erosion - in contrast to trickle of quiet, clear water - Thus, most erosion takes place during floods, when a stream carries a large volume of fast-moving, sediment-laden water.

erosion

- depends on the velocity of the flow - faster flows erode more rapidly = weaker substrate - not all substrate has the same resistance to this since ground is not perfectly planar - amout of vegetation that covers and protects the ground varies with location - carves valleys and canyons (shapes depnd on balance between slope collapse and down cutting rates)

river systems (streams)

- drain landscape of surface runoff - gradually erode landscapes and carry away debris - in result, no renewed uplift and mountains evolve into gentle hills - with them, can bevel once-rugged mountain ranges into nearly flat plains

annual probability

- flooding indicates likelihood that flood of given size or larger will happen at specific locality during any given year - ie. flood has 1% then we mean there is a 1 in 100 chance that flood of at least this size will happen in any given year

Permanent streams

- flow all year long - permanent streams exist where the floor (or bed) of the stream channel lies below the water table - In these streams, which occur in humid or temperate climates, water comes not only from upstream or from surface runoff, but also from springs through which groundwater seeps - if it lies above water table, then stream can be permanent only when rate where water arrives from upstream exceeds rthe rate at which water infiltrates into ground below

waterfall

- forms where the gradient of a stream becomes so steep that some or all of the water literally free- falls above the streambed - energy of falling water may scour depression - may appear to be permanent feature of landscape, they'll eventually disappear because headward erosion slowly eats back the resistant ledge - ie. Niagra Falls

overuse of water

- growing populations lead to increase in demand for this commodity - 65% of the water taken out of rivers is used for agriculture, 25% for in- dustry, and 9% for drinking

gravel bars

- if choked with sediment, may split into numerous enterwined channels separated from one another by this - creates braided stream

meandering stream

- if stream isn't choked by sediment flows over flat ground, it becomes this type of stream - winding back and forth in snake-like curves - changes shape over time with erosion and deposition - may be cut off creating oxbow lake

existence of several overlapping deltas

- indicates main course of the river in the delta has shifted on several occasions - shifts occur when a toe builds so far out into the sea that the slope of the stream becomes too gentle to allow river to flow

deep trough

- land surface lies well above the base level, a stream can carve this - much deeper than channel itself - can develop valley or canyon

V-shaped valley

- landform with cross sectional shape - stream channel lies at floor of valley - resembles letter V - when mass wasting takes place as fast as downcutting occurs

water table

- level below which groundwater fills all the pores and cracks in subsurface rock or sediment - Above this, air partially or entirely fills the pores and cracks

ephemeral streams (water table above)

- lies above water table - stream flows only when water enters stream faster than it can infiltrate into the ground

peneplain

- lies at an elevation close to that of a stream's base level - low hills are beveled down - becoming small mounds or even disappearing - rarely develop before downcutting begins again

groundwater sapping

- locations where groundwater seeps out of the ground and enters the entrance to the stream channel - gradually weakens and undermines soil or rock just upstream of channel's endpoint until the material collapses into the channel - collapsed debris eventually washes away during a flood - each increment of collapse makes the channel longer

base level

- lowest elevation a stream channel's surface can reach at a locality - limits the depth to which a stream can downcut its channel - different types: local, ultimate

ultimate base level

- lowest possible elevation along the stream's longitudinal profile - ie. sea level

oxbow lake

- meander may be cut off - leaving curving lake

saltation

- multitude of grains bounce along in the direction of flow - within a zone that extends up from the surface of the streambed for a distance of several centimeters to several tens of centimeters - Each grain in this zone follows a curved trajectory up through the water and then back down to the bed - When it strikes the bed, it knocks other grains upward, and thus supplies grains to the this zone.

meander neck

- narrow isthmus of land separating the portions of the meander - With continued erosion, a meander may curve through more than 180 , so that the cut bank at the meander's entrance approaches the cut bank at its end

Stream piracy

- natural process that happens when headward erosion by one stream causes the stream to intersect the course of another stream - pirate stream "captures" the water of the stream that it has intersected, so that the water of the captured stream starts to flow down the pirate stream - stream capture changes water gap into dry wind gap

tributaries

- new side channels - begin to form after downcutting deepens main channel while surrounding land surfaces start to slope toward the channel - flow into main channel - eventually linked streams evolves with smaller __________ flowing into trunk stream

local base level

- occurs at a location upstream of a drainage network's mouth - do not last forever because running water eventually removes obstructions that create them - ie. lakes and reservoirs along a stream, ledge of resistant rock, tributary joins larger stream

Stream rejuvenation

- occurs when a stream starts to downcut into a land surface - elevation lies near the stream's base level - can be triggered by several phenomena: drop in base level, uplift event, increase in stream discharge - can lead to formation of stream terraces in alluvium - sometimes causes a stream to erode deeply into the land, a new canyon or valley will develop - If the stream had a meandering course, downcutting produces incised meanders

flood

- occurs when volume of water flowing down stream exceeds volume of stream channel - water rises out of normal channel and spreads over floodplain or delta plain, or fills canyon to greater depth than normal - when the supply of water entering a stream exceeds the channel's capacity - water spills out and covers the surrounding land - 2 types: seasonal, flash

Parallel

- on a uniform slope, several streams with parallel courses develop simultaneously - group comprises a parallel network

drainage reversal

- overall slope direction of the drainage network became the opposite of what it once had been - ie. uplift of the Andes

depression

- plunge pool - base of waterfall

flood-hazard maps

- predict extent of land that will be submerged by flood - able to do so by knowing the discharge during flood of specified annual probability, shape of river channel, and elevation of land bordering the river

downcutting

- process where extra flow deepens the channel relative to its surroundings - then stream forms - deepens main channel while surrounding land surfaces start to slope toward the channel

sediment sorting

- process where stream deposits tend to be segregated by size - gravel accumulates in one location and mud in another

gravity

- pulls surface water (including meltwater) downhill into stream channels - pressure exerted by the weight of new rainfall squeezes existing soil moisture back out of the ground - groundwater seeps out of the channel walls into the channel if the floor of the channel lies below the water table

Breaking and lifting

- push of flowing water can break chunks of solid rock off channel floor or walls - flow of current over clast can cause clast to rise, or lift off substrate

effects of agriculture on streams

- rains in naturally vegetated regions where much water either soaks into ground or gets absorbed by plants - soil moisture or groundwater eventually seeps into nearby stream - remainder flows underground that results in water that reaches nearby streams after a storm less than total amount of precipitation - lag between water falling and stream's dischrage

stream's average discharge

- reflects the size of its drainage basin and the climate - stream varies along its length, human discharge, discharge at given location - ie. Amazon River drains huge rainforest with largest average discharge in the world at about 200,000 m3/s or 15% of total amount of runoff on Earth

Bird's-foot deltas

- resemble the scrawny toes of a bird - develop where several distributaries extend far out into relatively calm water

Dissolution

- running water dissolves soluble minerals as it passes - carries minerals away in solution

fluvial deposits (alluvium)

- sediments transported by a stream - may accumulate along the stream bed in elongate mounds, called bars

continental divide

- separates drainage that flows into one ocean from drainage that flows into another - ie. Rocky Mountains in US

friction

- slows the water on the floodplain, so a sheet of silt and mud settles out to comprise floodplain deposits

cutoff

- straight reach - when erosion eats through a meander neck

bed load

- stream consists of large particles (such as sand, pebbles, or cobbles) that bounce or roll along the stream floor - movement commonly involves saltation - coarster clasts may bounce and tumble here

stream terraces

- stream's base level later drops again and/or the discharge increases - stream will start to cut down into its own alluvium - bordering present floodplain

mountains

- streams carve deep, V shaped valleys - tend to have steep gradients

sheetwash

- thin film - some flows downslope - some sink into the ground where it either becomes trapped in soil (as soil moisture) or descends below the water table to become groundwater - velocity of this thin film varies with location

capacity

- total quantity of sediment it can carry - larger river = more ________ than small creek - depends on.... 1. competance 2. discharge

rapids

- turbulent water with a rough surface - form where water flows over steps or large clasts in the channel floor, where the channel abruptly narrows, or where its gradient abruptly changes - turbulence creates eddies, waves, whirlpools that roil and churn water surface - creates whitewater

turbulence

- twisting, swirling motion of fluid - can create eddies where water curves and flows upstream or circles in place - develops because the shearing motion of one water volume against its neighbor causes the neighbor to spin - because obstacles, such as boulders, deflect water flow - can make measuring velocity difficult

seasonal floods (wet season)

- typically take place in tropical regions during monsoons and temperatures during spring when storms drench land frequently and/or heavy winter snowpack melts - when rainfall is heavy or when winter snows start to melt - can submerge over floodplains (floodplain floods) and delta plains (delta plain floods)

suspended load

- usuallyconsists of tiny solid grains (silt or clay size) - swirl along with the water without settling to the floor of the channel - finer clasts rush along this

creation of meanders

- variations in water depth and associated friction causes fastest moving current to swing back and forth - water erodes side of stream more effectively where it flows faster - cutting away faster on outer arc of curve - each curve begins to migrate sideways and grow more pronounced until it becomes this - outside edge = erosion continues to eat away at channel wall - inside edge = water slows down so competence decreases and sediment accumulates, forming point bar

water velocity

- varies across streams - tends to be turbulent - calculating average _________ is complicated

discharge

- volume of water passing through cross section of stream in given time - CALCULATE: D = Ac X va

alluvial fan

- water slows and drops its sedimentary load - forming a sloping apron of sediment - stream then divides into a series of small channels that spread out over the fan

running water

- water that flows down the surface of sloping land in response to the pull of gravity - collects in stream channels

lower floodplain

- when base level falls or discharge increases - stream downcuts through alluvium - remanants of original alluvial plain remain as pair of terraces

stair-step shape

- when walls of stream consist of alternating layers of hard and soft rock - downcutting trough alternating hard and soft layers - ie. Grand Canyon

delta

- where a stream empties at its mouth into a standing body of water - water slows and a wedge of sediment accumulates - any wedge of sediment formed at a river mouth - some are smooth, elogated lobes, etc. - build into standing water

drainage network types (several types of networks on the basis of the network's map pattern)

1. Dendritic 2. Radial 3. Rectangular 4. Trellis 5. Parallel

erosion types from running water

1. Scouring 2. Breaking and lifting 3. Abrasion 4. Dissolution

pothole

abrasion by sand or gravel carves a bowl-shaped depression in floor of stream

ways to characterize risk of flooding

1. annual probability 2. recurrence interval

channel formation

1. begins when sheetwash starts flowiing downslope that erodes substrate (material it flows over) 2. round is not perfectly planar, not all substrate has the same resistance to erosion, and the amount of vegetation that covers and protects the ground varies with location 3. velocity of sheetwash also varies with location 4. Since the channel is lower than the surrounding ground, sheetwash in adjacent areas starts to head toward it 5. With time, the extra flow deepens the channel relative to its surroundings, a process called downcutting, and a stream forms

ways to describe stream's ability to carry sediment

1. competance 2. capacity

living with floods

1. control (artifical levees) 2. evaluating flood hazard (annual probability, recurrence interval)

ways stream carries sediment

1. dissolved 2. suspended 3. bed loads

Stream rejuvenation causes

1. drop in base level: happens when sea level falls 2. uplift event: land rises relative to the base level 3. increase in stream discharge: makes the stream more able to erode and transport sediment

hydrologic cycle

1. evaporating from Earth's surface and rising into atmosphere 2. condenses and falls back onto Earth's surface as rain or snow

factors affecting shape of delta

1. form where the ocean currents are strong have a shape for the ocean currents redistribute sediment in bars running parallel to the shore 2. places where waves and currents are strong enough to remove sediment as fast as it arrives a river has NONE

differences in average velocity of stream water (va) depends on these factors

1. friction along the sides = floor of the stream slows the flow 2. turbulence

landforms along meandering streams

1. natural levees 2. point bars 3. floodplains

environmental issues pertaining to rivers

1. pollution 2. dam construction 3. overuse of water 4. effects of urbanization and agriculture on streams

sediment

1. sand 2. gravel 3. cobbles)

types of floods

1. seasonal 2. flash

when flow velocity decreases

1. slope of streambed becomes shallower 2. channel broadens out and friction between bed and water increases, then competence of stream decreasess and sediment settles out

variations (stream's average discharge)

1. stream varies along its length 2. human discharge 3. discharge at a given location can vary with time

types of streams that cut across resistant topographic highs

1. superposed streams 2. antecedent streams

where rapids form

1. water flows over steps or large clasts in the channel floor 2. channel abruptly narrows 3. gradient abruptly changes

reasons headward erosion occurs

1. when the surface flow converging at the entrance to a chan- nel has sufficient erosive power to downcut 2. locations where groundwater seeps out of the ground and enters the entrance to the stream channel

seasonal floods

submerge floodplains and delta plains at certain times of the year

sediment load

total volume of sediment carried by a stream includes.... 1. dissolved load 2. suspended load 3. bed load

streams mouth

cannot be lower than its base level

piracy

channel of the captured stream, downstream of the point of capture, dries up

atmospheric water

condenses and falls back to the Earth's surface as rain or snow that accumulates in various reservoirs

channel

elongate depression or trough

stream-carved landscapes

evolve over time as gradients diminish and ridges between valleys erode away

abandoned meander

if it remains filled with water, then it creates this dried out meanders

reach

interval along the length of a stream

bars

may accumulate along the stream bed in elongate mounds

oxbow lake

meander that has been cut off

Earth

only planet in the Solar System that cur- rently hosts flowing streams

cut bank

outside edge = erosion continues to eat away at channel wall

Scouring

process of running water that can remove loose fragments of sediment

avulsion

river overflows a natural levee upstream and begins to flow in a new direction

drainage divide

separates one watershed from another