Landform Geography Landforms of the Fluvial System.

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