Geomorphology Exam 2
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.