landform geography
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Landform Geography
Landforms of the
Fluvial System
Stream GradationStreams evolve to carry just the amount of
sediment produced by their drainage basin
Meandering StreamStream carrying mostly suspended load
usually has deep channel curving side-to-side
Braided Stream
Streams with mostly bed load will have braided appearance – wide, shallow channel
Graded Stream
A stream with just enough discharge to carry its average sediment load – at equilibrium
Aggradation – deposition on the bed of the channel due to added load or decreased discharge
Degradation – erosion of channel bed due to increased discharge or decreased load
Evolution of a Graded Stream
Base Level – lowest level at which a stream can erode its channel bed – can rise & fall with changes in sea or land level
Niagara Falls
Upstream (Lake Erie) 571 ft elevDownstream (Lake Ontario) 243 ft elevHas retreated almost 7 miles in past 12,000 yrs
Stream Meandering
Oxbow Lake
Floodplain Features
Drainage Basin Changes
Alluvial FansCreated by alluvial aggradation in areas of high relief
where bedload-dominated streams flow out of mts onto plain – stream sweeps side to side over time, making fan shape – most common in desert climates
Deltas
River water slows as it enters ocean, depositing large amts of sediment in triangular pattern called delta – smaller particles farther out in delta
Mississippi River
Human Interaction with StreamsUrbanization
More impervious surfacesAfter heavy rain, shorter lag time, higher peak flow
– more serious flooding
Dams and ReservoirsDam – engineered obstruction across a river to
control its flow – usu. holds back lake or reservoir
Why dams?1. Hydroelectric energy2. Flood control3. Enhance river navigation4. Surface water supply
Hoover Dam
Chattahoochee and Buford Dam
Negatives of Dams
Expense
Refugees from areas flooded by dam
Loss of ecosystems & scenery
No floods/No soil replenishment in floodplain
Stop spawning fish from going upstream
Overland Flow
Perennial Streams – water runs all year
Ephemeral Streams – water runs only part of year
Sources of stream water:
Groundwater
Melting Ice
Surface Runoff
Drainage BasinsDrainage Basin – area contributing
groundwater and runoff to a stream Drainage basins are divided from each other by
topographic barriers called watersheds
Drainage Divide – elevated terrain forming rim around a drainage basin
Drainage basins vary tremendously in size
Basins are nested, smaller within larger
Small, Nested Drainage Basins
Common Drainage Pattern Types
Major US Watersheds
Drainage DensityTotal length of all streamsDrainage Density = Area of drainage basin
Stream OrderingRepresents stream sizeSmallest streams in basin order 1 – order rises when 2
streams of same order come together at confluence
Hydraulic GeometryGeometric attributes of river channelsVariables:
w = channel widthd = channel depthv = velocity of waters = slope (steepness) also called gradientQ = discharge (amt of water flowing)Q = w x d x v (units m3/s or ft3/s)
Hydraulic Variables
Stream HydrographGraph showing fluctuation in stream discharge over timeLag between storm event and highest discharge
• Base flow – flow rate sustained by groundwater influx
Flooding
Flood Stage – stream discharge increases so that water spills out of channel onto adjoining ground
Return Period – time between events of a given magnitude, e.g. annual flood, 50-year flood, 100-year flood
Larger floods occur less frequently
Mississippi River Floods - 1993
At least 100-year flood, perhaps a 500-year flood
Heavy winter rains saturated ground
Stationary high pressure in Southeast in summer, blocking mid-latitude jet stream over Midwest
Cool, dry air collided with warm, moist air along jet stream, creating constant precipitation
Precipitation ran off into stream channels & rivers
Mississippi River Floods - 1993
2009 Carrollton Floods
Fluvial Processes and LandformsRunning water is most important geomorphic (landform
shaping) process on Earth’s surface
All landforms due to either erosion or deposition
Erosional landforms occur when sediment, soil, or rock is stripped away from land
Depositional landforms occur where sediment accumulates after being dropped
Depositional vs. Erosional Landforms
HillslopesMost active zones of fluvial erosion due to high relief
creating fast-moving, powerful water
Rills Gully
Ravine
Canyon
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