Chapter 11Fluvial Geomorphology

StreamsStreamsThe work that streams do:The work that streams do:• Erosion/DenudationErosion/Denudation• TransportationTransportation• DepositionDeposition

Drainage SystemsDrainage Systems•Drainage system—A Drainage system—A

branched, hierarchical branched, hierarchical network of streams and network of streams and tributariestributaries

•Valley—Where a drainage Valley—Where a drainage system is clearly system is clearly establishedestablished

• Interfluve—Interfluve—(“inter”=between, (“inter”=between, “fluvia”= rivers) High “fluvia”= rivers) High ground that separates ground that separates valleysvalleys

•Drainage divide—The Drainage divide—The invisible line separating invisible line separating two drainage basinstwo drainage basins

Drainage basin / WatershedDrainage basin / Watershed•A single network system; includes both the A single network system; includes both the

channeled valley and any other land surface channeled valley and any other land surface contributing overland flow or groundwater to the contributing overland flow or groundwater to the streamstream

•Overland flow—unchannelized flow of waterOverland flow—unchannelized flow of water

Three Processes of Stream Three Processes of Stream ErosionErosion•Hydraulic action—The physical force of water Hydraulic action—The physical force of water

pounding on rocks and land materials, pounding on rocks and land materials, breaking them apartbreaking them apart

•Abrasion—Rock materials hitting the bed Abrasion—Rock materials hitting the bed (bottom) of the river and its banks (sides)(bottom) of the river and its banks (sides)

•Corrosion—The chemical action of water Corrosion—The chemical action of water dissolving minerals and rock materialdissolving minerals and rock material

How Quickly Erosion Occurs How Quickly Erosion Occurs Depends On…Depends On…

•Flow SpeedFlow Speed

•TurbulenceTurbulence

•Resistance of the bedrockResistance of the bedrock

1. Flow Speed1. Flow Speed

•Flow SpeedFlow Speed–The faster the water, the more force it hasThe faster the water, the more force it has–Faster water = more erosionFaster water = more erosion

What makes the water go What makes the water go faster?faster?

1. Steeper gradient (slope)1. Steeper gradient (slope)

2. Volume of flow (discharge)2. Volume of flow (discharge)

3. Channel width3. Channel width– The narrower the channel, the swifter the flow, The narrower the channel, the swifter the flow,

for the same volume of water (remember the for the same volume of water (remember the Venturi effect!)Venturi effect!)

Which has more erosive Which has more erosive force?force?

2. Turbulence2. Turbulence

• Turbulence is determined by:Turbulence is determined by:–Flow speedFlow speed

•Faster flow = increased turbulenceFaster flow = increased turbulence

–Roughness of the stream channelRoughness of the stream channel•A rough, irregular channel = more turbulenceA rough, irregular channel = more turbulence

3. Resistance of bedrock3. Resistance of bedrock

•Harder rocks = less erosionHarder rocks = less erosion

Transportation: Stream LoadTransportation: Stream Load•Stream load (rock material transported by Stream load (rock material transported by

streams) is carried in three forms:streams) is carried in three forms:

1. Dissolved load—dissolved minerals carried in 1. Dissolved load—dissolved minerals carried in solutionsolution

2. Suspended load—small particles that never 2. Suspended load—small particles that never touch the stream bed (touch the stream bed (Most of the stream load is Most of the stream load is suspendedsuspended.).)3. Bedload—larger rock fragments that drag, roll, 3. Bedload—larger rock fragments that drag, roll, skip or bounce along the stream bed skip or bounce along the stream bed

• Material is Material is picked up, picked up, dropped and dropped and picked up againpicked up again

DepositionDeposition

•Deposition occurs when either flow Deposition occurs when either flow speed or volume speed or volume decreasedecrease

•Conditions that cause deposition:Conditions that cause deposition:–Change in gradientChange in gradient–Channel wideningChannel widening–Flowing into less active waterFlowing into less active water–Change of directionChange of direction

Deposition: AlluviumDeposition: Alluvium•Alluvium—Any stream-deposited debrisAlluvium—Any stream-deposited debris

–Smaller particles are carried farther than Smaller particles are carried farther than large oneslarge ones

–Larger particles drop out of suspension firstLarger particles drop out of suspension first–The longer alluvial material is transported in The longer alluvial material is transported in

the stream, the more rounded and smooth it the stream, the more rounded and smooth it becomesbecomes

Stream FlowStream FlowWhat if the stream isn’t straight?What if the stream isn’t straight?

•PatternsPatterns– StraightStraight– SinuousSinuous– MeanderingMeandering– BraidedBraided

EquilibriumEquilibrium

• All factors are balanced (gravity, All factors are balanced (gravity, stream load, deposition, down-stream load, deposition, down-cutting)cutting)

• More theoretical than actual, because More theoretical than actual, because equilibrium is so difficult to achieve—equilibrium is so difficult to achieve—and maintainand maintain

What happens as a stream tries What happens as a stream tries to reach “equilibrium”?to reach “equilibrium”?

•Valley deepeningValley deepening

•Valley widening and flatteningValley widening and flattening

•Valley lengtheningValley lengthening

Valley Deepening:Valley Deepening:Knickpoint MigrationKnickpoint Migration

•Downcutting Downcutting progresses progresses upstreamupstream, until the , until the valley is all at one valley is all at one level level

Valley Deepening: Knickpoint Valley Deepening: Knickpoint MigrationMigration

Valley Widening and Valley Widening and FlatteningFlattening

The Role of Meanders

Valley Widening and Valley Widening and FlatteningFlattening

Meanders and Oxbow LakesMeanders and Oxbow Lakes

Can you find an oxbow lake?Can you find an oxbow lake?Where are cutoffs about to Where are cutoffs about to form?form?

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Structures of a FloodplainStructures of a Floodplain

Structures of a FloodplainStructures of a Floodplain

Valley Lengthening:Valley Lengthening:Headward ErosionHeadward Erosion

Valley Lengthening:Valley Lengthening:Delta Formation and Delta Formation and DistributariesDistributaries

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Stream Rejuvination: Rapid Stream Rejuvination: Rapid upliftuplift

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Entrenched meanders: Slow Entrenched meanders: Slow upliftuplift

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Entrenched meandersEntrenched meanders