physical geography lecture 16 - fluvial geomorphology 120516

Chapter 11

Fluvial Geomorphology

StreamsThe work that streams do:• Erosion/Denudation• Transportation• Deposition

Drainage Systems•Drainage system—A

branched, hierarchical network of streams and tributaries

•Valley—Where a drainage system is clearly established

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

•Drainage divide—The invisible line separating two drainage basins

Drainage basin / Watershed

A single network system; includes both the channeled valley and any other land surface

contributing overland flow or groundwater to the stream

Erosion•Overland flow—unchannelized flow of water– Splash erosion– Sheet erosion

•Hydraulic action•Abrasion•Corrosion

Three Processes of Stream Erosion•Hydraulic action—The physical force of water pounding on rocks and land materials, breaking them apart

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

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

How Quickly Erosion Occurs Depends On…1. Flow Speed2. Turbulence3. Resistance of the bedrock

1. Flow SpeedFlow Speed

–The faster the water, the more force it has–Faster water = more erosion

Factors influencing flow speed1. Steeper gradient

– Gradient—The rate of fall in elevation of the stream surface in the downstream direction (as in 20 ft./mi.)• Steeper gradient = faster flow = more erosion

2. Volume of flow (discharge)– More water = higher speed = more erosion

• Flood events move bigger material—and more of it!3. Channel width—the narrower the channel, the

swifter the flow, for the same volume of water (Remember the Venturi effect?)

How much erosive force?

How much erosive force?

How much erosive force?

2. Turbulence

•Water swirling and tossing, not smooth flow

•Turbulence is determined by:–Flow speed

•Faster flow = increased turbulence–Roughness of the stream channel

•A rough, irregular channel = more turbulence

3. Resistance of bedrockHarder rocks = less erosion

Transportation: Stream LoadStream load (rock material transported by streams) is carried in three forms:

1. Dissolved load—dissolved minerals carried in solution2. Suspended load—small particles that never touch the stream bed (Most of the stream load is suspended load)3. Bedload—larger rock fragments that drag, roll, skip or bounce along the stream bed Material is picked up, dropped and picked up again

Transportation: Stream Load

Transportation: Competence• The faster the stream is flowing, the

larger the particles it can transport• This measurement is called

competence– Competence varies to the sixth power of the

water’s speedIf flow speed doubles:

26 = 64 times the size!– This is why flood events are so significant in

stream transportation

Transportation: Capacity•Capacity—A measure of the amount of solid material potentially transported (volume/time past a given point: gal/sec)

•The capacity of a given stream depends on:

1. Water volume(volume↑, capacity↑)

2. Flow speed(flow speed↑, capacity↑)

3. The type of load material(lighter material, capacity↑)

Deposition•Deposition occurs when either flow speed or volume decrease

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

Deposition: Alluvium•Alluvium—Any stream-deposited debris

–Smaller particles carried farther than large ones•Sorting—Based on size. As stream flow decreases, larger particles drop out of suspension first

•Shape—The longer alluvium in transported in the stream, the more rounded and smooth it becomes

Sorting after a flood event

(particles settle biggest first, smallest last)

erosion deposition

Well-rounded alluvial deposits

Stream FlowHow does friction from the channel sides and bottom affect stream flow?

Stream Flow•Patterns

–Straight–Sinuous–Meandering–Braided

Just like a lake, water balance must be maintained year-round, or the stream will

become ephemeral, only in existence for part of the year.

Equilibrium: A Graded Stream• All factors are balanced (gravity,

stream load, deposition, down-cutting)

• A graded condition is more theoretical than actual, because equilibrium is so difficult to achieve—and maintain

Which processes happen as a stream tries to attain a graded condition?•Valley deepening•Valley widening and flattening•Valley lengthening

Valley Deepening:Knickpoint Migration•Downcutting progresses upstream, until the valley is all at one level

Valley Deepening: Knickpoint Migration

Valley Widening and Flattening

Valley Widening and Flattening

The Role of Meanders

Meanders and Oxbow Lakes

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

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Structures of a Floodplain

Structures of a Floodplain

Aggradation—Deposition that raises the stream bed

Valley Lengthening:Headward Erosion

Valley Lengthening:Delta Formation and Distributaries

Valley Lengthening:Delta Formation and Distributaries

Valley Lengthening:Delta Formation and Distributaries

Valley Lengthening:Delta Formation and Distributaries

Stream Rejuvenation

Degradation—Erosion that lowers the stream bed

Stream Rejuvenationand Entrenched Meanders

When uplift is slow, meanders cut down into their existing channels and cutoffs form arches.

Entrenched Meanders