Geomorphology Exam 2

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Lane's Balance

(degradation) sediment discharge x sediment size = water discharge x water slope (aggradation) if out of balance aggradation and degradation

stage

(height of water above a datum)

bankfull width

Channel width at bankfull discharge

Nearly all soil that is eroded off hillslopes is quickly transported by rivers T or F

F

meandering river occur in ... causing

In wide valleys causing lower sediment load which makes it not become a braided stream.

flow velocity increases with distance down stream T or F

T

in turbulent flow the water column is more mixed T or F

T

there is more water in channels when there is a higher drainage density T or F

T

bankfull

The maximum discharge that a river channel is capable of carrying without flooding.

suspended sediment

Very small particles which remain distributed throughout the water column due to turbulent mixing exceeding gravitational sinking. gives water its color

slopewash

Water that doesn't infiltrate flows across the surface as runoff, advective sediment transport (movement by flowing water

natural levee

a form of vertical accretion. a deposit of sand or mud built up along, and sloping away from, either side of the flood plain of a river or stream

drainage divide

a highland or ridge that separates one watershed from another

competence

ability if flow to transport particles of given size

on the outside of the bend

accelerating downward flow, scours the Botton forming pools and cutbacks

where is sediment yield highest

actively lifted mountain ranges with orographic precipitation. mountain ranges with large glaciers semiarid regions covered by loess

The critical distance is where

advective processes begin to dominate and the channel head forms

concave lower slope is a result of .... why?

advective/convergent processes (surface runoff) If slope did not decrease as runoff increases with distance from the divide, then sediment transport capacity would exceed the quantity of sediment supplied from upslope

Cut and nested fill terraces form by

aggradation and incision as river is working toward equilibrium

If a graded stream is not equilibrium

aggradation or degradation

drainage basin

an area that supplies water and sediment to a channel. includes hillslopes and channels

drainage network

an array of interconnecting streams that together drain an area

drainage area

area within drainage divide

splays are frequently located

at breaks or low areas in natural or artificial levees (crevasse).

in braided stream there are frequent

bar dissection and channel avulsions (abrupt changes in channel position)

coarser deposits from splays are primarily

bedload

Raindrop impact on bare soil _________ soil aggregates and forms ______ ___ ________ _______ _______which __________infiltration capacity up to _____

breaks , seal of small particles, decreases, 50%

What types of environmental change might lead to aggradation and degradation

building a dam- raises base level leads to deposition removing a dam- lowers base level leads to incision

where is fastest flow in a river

center near surface

at the apex of the bend

centrifugal force pushes water to the outside, eroding it, and the inwards secondary flow deposits it on the inside

Graded stream longitudinal profile is shaped_______ because

concave up upper reach leads to low Q and high friction leads to high gradient required to maintain sediment transport capacity lower reach leads to high discharge and higher friction leads to high gradient NOT required to maintain sediment transport capacity

rating curve

converts stage to discharge

inside of the bend

decelerating upward floes, deposition, creating point bars

sediment yield tends to _______ with increasing basin size because...

decrease Small (low order)basins → steep valley side and high gradient streams → high sediment supply and little sediment storage Large basins (high order)→ wider floodplains, valleywalls further from river, greater sedimentstorage

Soil compaction _______ infiltration rate

decreases

as material moves down hill average size

decreases

as stream order increases slope _____

decreases

what happens to channel roughness as you move downstream

decreases.

Thalweg

deepest part of the channel

stream power

determines capacity of stream to transport sediment. function of density of water, acceleration of gravity, discharge and bed slope.

hydrographic

discharge plotted agains time at a point along a stream.

effective discharge

discharge that transports the largest fraction of the sediment load over a period of years.

why do diffusive processes create a convex upper slope?

divergent, Each section of hill slope contributes mass that moves downslope → soil flux must increase downslope → slope must steepen to transport increasing amount of material

subsurface flow

downslope movement of water in soil macropores. more likely to happen in deep. permeable soils with a steep straight/convex hillslope in a humid climate with dense vegetation.

a ten year flood is

equalled or exceeded every ten years on average.

Splays

fan-shaped deposit on floodplain located where fast deep water spilled out of channel.

What effect do humans have on sediment yield

farming and urbanization/construction increases sediment yield. Poor land management resulted inwidespread erosion on agricultural fieldsand sediment deposition on downstream flood plains.

for meandering rivers radius of curvature typically 2-3 times bankfull width due to

feedback between flow through bend and bank erosion.

need ______ ________ to cut definable channel

flow concetration

flow irregularities in straight channels lead to

formation alternate bars and pools

smaller material will be found where compared to river channel

further from the river channel.

where does soil form in humid landscapes

gentler slopes

channel threshold line varies dependingon

geology, climate, landcover

sediment yield is very high down stream of what

glaciated basins

diffusive processes

gravity driven sediment transport (rain splash, creep, tree throw, burrowing) and the rate depends on slope steepness. dampen out perturbations smoothing topography. creates a convex upper slope.

for braided streams channel width ______ compared to depth.

high

high drainage density is associated with _____ peak discharge

high

high soil/rock erodibility does what to drainage density and critical distance

high drainage density and small critical distance

grass covered sand dunes have a _______ infiltration rate which causes _________ ________ and _________ flooding and erosion

high, groundwater recharge, minimal

The lower the antecedent moisture the _______ the infiltration capacity

higher

High rainfall does what to drainage density and critical distance

higher drainage density and a small critical distance

urban areas have _________ peak discharge _________ rate of increase _________ storm flow _________ base flow _________ groundwater recharge

higher peak discharge higher rate of increase higher storm flow lower base flow lower groundwater recharge

Braided channels need _________ ___________ discharge because it causes....

highly variable, bank erosion and irregular bed load movement.

Braided channels need _______ erodible banks with low ______ and/or little_________ because ...

highly, low cohesion, little vegetation, .... it enables the widening of channel and banks provide sediment

Why are natural levees made of coarsest material

immediate reduction in depth at edge of channel leads to reduction in transport capacity which leads to rapid deposition of coarsest material. this makes it have higher edges and forms more of a channel.

as base levels decrease streams ________ as base levels increase streams _______

incise deposit

entrenched meanders

incised river meanders excavated deeply into the landscape. meanders developed while land is flat. stays at same elevation by incising against upward movement of land and stops meandering. Ex: Colorado river incised into bedrock.

increasing runoff and uaseflow does what to discharge

increase

what happens with distance downstream to particle roundness particle size and particle resistance

increase decrease increase

seepage erosion undercuts slope causing slope to ________ and cause _______ ______ to occur which causes the channel to advance _____________

increase, mass wasting, upslope

increasing relief causes ______ slope steepness and thus _____ drainage density

increased, increased

clear cutting trees causes ______ runoff and ______ channelization threshold

increased, lower

conversion to pasture and row crops causes ______ runoff and thus ________ channelization threshold

increased, lower

increase in rainfall causes _______ runoff which causes _______ channelization threshold

increased, lower

In general sediment discharge in rivers _________ as river discharge increases

increases

Well-developed soil structure _______ infiltration rate

increases

as slope steepness increases, the shearing stress of a material

increases

as stream order increases discharge ________

increases

what happens to stream slope with distance down stream

increases

in humid regions as drainage basin area increases, stream width stream depth stream cross area stream discharge stream gradient flood plain width sediment storage

increases increases increases increases decreases increases increases

Dense vegetation and litter layer ________ infiltration rate

increases infiltration rate

As drainage area increases flow depth _________ causing shear stress to __________

increases, increase

as discharge increases depth ______, width _______, and velocity _________.

increases, increases, and increases.

The larger and more connected the pores, the greater the

infiltration rate larger particles tend to have a faster infiltration rate.

local convexities are found at

knickpoints Ex: where bed material is locally more resistant (small layer of harder rock in a softer rock)

local base level (lakes rivers confluences) can cause

local convexities in the longitudinal profile

ultimate base level versus local base level

local is lakes or streams while ultimate is the sea. water wants to go as low as possible, ultimate lowness vs local lowness

Floodplain

low area adjacent to stream channel, composed of sediment (alluvium) that is in transit. temporary storage occurs on the floodplain. actively build by lateral and vertical accretion of sediment.

Flood basin

low area located at a distance from channel.

lots of vegetation does what to drainage density and critical distance

low drainage density and large critical distance

agriculture on silt causes ______ infiltration, _______ surface runoff and _________ flooding and erosion

low, high, frequent

degradation

lowering of rive and parts of flood plain

Weathering-limited hillslopes

material transported away at faster rate than produced develop a faceted or angularmorphology; contributes debris to a lower slope Common in arid and high alpine environments. Bare, rocky landscape Mass wasting is an important process

Transport limited slope

material transported away at slower rate than produced → weathered sediment remains on the surface (i.e., soil) Rounding of hillslopes result fromdiffusive and advective processes

Bankfull discharge

maximum Q that can be contained within the channel

which location on a stream experiences the highest amount of discharge during a storm

middle location bc moving fastest.

what types of floods do the most work and maintain the channel size

moderate floods

The largest floods move ____ material and happen _______ The smallest flood move _____ material and happen________

more, infrequently less, frequently

Anastomosing channels

multiple sinuous channels with cohesive, vegetated banks that limit channel widening. Have avulsions created during over-bank flow cut new channels in floodplain. often found in low gradient(flat) areas where deposition diverts flow in two directions.

flood plain is constructed from

not necessarily during flooding: lateral erosion and deposition overbank flow/flood: vertical accretion forms a natural levee

centrifugal force elevates water surface __________ __ ______ which causes __________ ______________ ____________

outside of bend, helical secondary flow

flood plain land forms

oxbow lake- water in it Natural levee abandoned channel-oxbow lake with no water in it flood basin meander scrolls-terraces from former channel path splays

Seepage erosion

particle eroded as water emerges from highly permeable layer located above low permeability layer

what is flood lag in a hydrograph

peak is after precip not during because of runoff and basin characteristics.

Advective processes increase

perturbations, decreasing smoothness

sediment capacity

potential volume of sediment that can be transported by a given flow

sediment yield

quantity of sediment supplied to drainage basin over time measured as mass of sediment per unit area of drainage basin per unit time

saturation overland flow

rain on saturated soil or water table rises to surface. most common at base of hillslopes. can occur at lower rainfall intensities than infiltration excess overland flow. normally in thin soils at gentle concave footslopes in humid climates with dense vegetation.

Infiltration Excess (Horton) Overland Flow

rainfall rate exceeds infiltration rate More common in arid climates and onhuman disturbed land. in medium thickness soil on straight slopes.

local relief

range in basin elevation/basin area

sinuosity

ratio of channel length to the straight line distance measured down the valley axis. measures snakiness.

How is slope angle affected by the restsistance of material

resistance of underlying rock to collapse."Stronger" rocks (limestone &sandstone)→ steeper slopes"Weaker" rocks (shale)→ gentlerslopes

fluvial processes are associated with

rivers or streams

in humid, temperate areas bankfull area

scaled to contain floods with recurrence interval of 1-2 years

channel patterns arise from balances between

sediment supply and transport capacity. (gradient, bank material, floodplain vegetation, bedrock structure, and climate are also important factors)

bed load

sediment that is carried by a stream along the bottom of its channel

dissolved load

sediment that you cannot see in the water

which climates have higher sediment yield

semi arid because they have more precipitation but little vegetation.

horton model

shear stress increases with distance from divide at critical distance, she stress = critical shear and channel formation begins as flow cuts through soil and vegetation

What type of material are flood basins

silts and clays deposited by still waters

straight channel

sinuosity <1.3, rare, found in-between fields and in cities made of concrete.

meandering river

sinuosity >1.5. most common natural channel form. Deep narrow channel with few islands.

Surface runoff becomes more important as

slope steepens and contributing areas increases

The smaller the pores the greater the

soil moisture holding capacity due to greater surface area and tension

the channel gradient in braided streams is generally ________ than meandering rivers.

steeper

finer materials are primarily

suspended load

incision of Mississippi lowered local base level for tributary streams causing them to incise and export sediment which causes

terraces formed in tributary valleys during holocene which caused valley aggradation later human land use caused more aggradation

Rock slopes found in humid landscapes where

terrain is steep

why is there no vegetation on braided stream banks

the highly variable discharge makes it hard for vegetation to grow.

porosity

the percentage of material that is void space Greater porosity does not mean greater infiltration

How should reoccurrence intervals be interpreted

the probability of a flood with a given reoccurrence interval being exceeded within the next n years is q = 1-[1-(1/R)]^n

Soil infiltration capacity

the rate water can be absorbed into the so Factors influencing infiltration capacity: • Antecedent soil moisture • Soil texture • Soil structure • Soil compaction • Vegetation cover

Quantity of sediment transport by advective processes (surface runoff) proportional to

the slope steepness AND the quantity of runoff

Alluvium

the sorted material deposited by a stream

why do braided streams have high bed load

they lack capacity to transport or lack of competence to move the size of the sediment causing the stream forced to flow around its own sediment at low flows.

sheet wash

thin sheet of unchannelized water flowing over land smooths does not channelize

drainage density

total length of channel in a basin divided by area of basin

Equilibrium in a graded stream

transport capacity = sediment supply

braided stream

unvegetated bars, flow divided around coarse grained, unstable bars. abundant bed load.

aggradation

upbuilding of channel bed and floodplain

what is the dominant source of sediment supply to lowland streams

upstream channels and bank erosion

flood basin is formed from

water ponding up during and after floods

Annual flood frequency curves are useful when?

when determining the magnitude of the floods

for meandering rivers, wavelength and amplitude increase with

width and discharge.


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