Channel NetworksChannel Networks The drainage basin, or watershed, is the surface around a channel that "sheds water and sediment into that channel"

Streams that get their water from overland or groundwater flow, and not from other streams, are first -order streams

A second-order stream begins at the junction of two first order streams

If two second-order streams join, a third-

order stream is formed

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Stream PowerStream Power

• Critical Power is that rate of useful energy necessary to transport the existing [sediment] load

• If more Kinetic Energy, erosion (degradation) will result

• If there is too much sediment for the available energy, deposition (aggradation) will result

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Driving and Resisting ForcesDriving and Resisting Forces• Driving forces: gravity Driving forces: gravity

propelling water downslopepropelling water downslope• Resisting forces: friction within Resisting forces: friction within

the fluid (water), and friction the fluid (water), and friction between water and the between water and the channel boundarychannel boundary

Conversion of Potential Energy into Kinetic Energy

Conservation of Mass (Continuity)Conservation of Mass (Continuity)The Role of Cross Section AreaThe Role of Cross Section Area

• Mass is neither created nor destroyed

• Inputs = outputs• Inputs and outputs for fluid

flow are discharge

Vel (V) x Area(A) = const.

V1A1 = V2A2

If area gets smaller, velocity gets larger

Gilbert’s Fluvial ProcessGilbert’s Fluvial Process

• Joined John Wesley Powell survey in Utah, 1874

• First suggested the concept of “graded streams”

• A stream’s form is defined by its ability to transport load, and that a “graded” stream condition will exist when the stream can just carry the load supplied to it– “The transportation of debris by running water”,

USGS Prof. Paper 86, Grove Karl Gilbert, 1914

Grove Karl Gilbert

If a stream …reaches a point where the slope is less, it becomes overloaded and part of the load is dropped,making a deposit.”

“If a … stream reaches a point where the slope is steeper …it [goes faster and erodes] more load...”

Graphic by Peter WilcockText by G.K. Gilbert, “HydraulicMining Debris in the Sierra Nevada”USGS Prof. Paper 105, 1917.

Gilbert: The Role of Stream Slope

Sediment Sources, Storage, and YieldsSediment Sources, Storage, and Yields

• Most stream sediment from Most stream sediment from creep, soil erosion, mass wasting creep, soil erosion, mass wasting

• Called “Colluvium”Called “Colluvium”• River incision of bed-rock important after dramatic increase in River incision of bed-rock important after dramatic increase in

elevation above sea-level.elevation above sea-level.• Steep streams form vee-shaped channelsSteep streams form vee-shaped channels• Most streams transport limited: supply exceeds removal ability. = Most streams transport limited: supply exceeds removal ability. =

"Alluvial Streams""Alluvial Streams"

Most streams store sediment, only about 5 to 7 % of sediment Most streams store sediment, only about 5 to 7 % of sediment input removed by main trunk.input removed by main trunk.

• Meandering covers bed load with point bar and floodplain Meandering covers bed load with point bar and floodplain deposits. deposits.

6_5

Particles are large andirregular, and consist ofa variety of minerals,including the leastresistant.

Particles are mid-sizedand of intermediateroundness, and includeresistant and nonresistantminerals.

Particles are small andnearly spherical, andconsist mainly of themost resistant minerals,such as quartz.

Character of detrital sediments depends on time, distance, and energy. For example, in streams:

HIGHLANDS LOWLANDS NEAR-COASTAL

Downstream Changes Downstream Changes in Alluvium Grainsin Alluvium Grains

Active and Relict AlluviumActive and Relict Alluvium

See Figure 11-8See Figure 11-8

Active: Active: that sediment that that sediment that can be removed and can be removed and carried by the stream that carried by the stream that now covers it.now covers it.

Anything deeper Anything deeper or coarser is or coarser is relictrelict

Floodplain MorphologyFloodplain Morphology

oxbow

pointbar

cutbank

chuteyazoo

Splay

Scrollbars

colluvium

floodplain

floodplainfloodplain meander scar

Splay : pile of sediment on floodplain from a levee breach

Meander scar a crescentic cut in a bluff or valley wall produced at a cut bank

Scroll-bars (aka Scroll marks) an asymmetrical ridge and swale topography on the inside of the bends due to loop migration.

Fluvial DepositsFluvial Deposits

Table 11-2

Loop Cutoff and Oxbow FormationLoop Cutoff and Oxbow Formation

Figure 11-10

1949 1958

Floods allow shortcuts. A shortcut is a short lateral path for the same drop, i.e. a steeper gradient. Shortcuts lose less energy to friction, and they cause oxbows.

Scroll marks, point bar scars

Crevasse Splay

Skip Climatic Change and Aggradation.

Terraces

• Remnants of former floodplainRemnants of former floodplain– River adjusts to uplift or drop in ultimate River adjusts to uplift or drop in ultimate

base level (sea-level) by downcuttingbase level (sea-level) by downcutting– Any spot is higher, water falls further to Any spot is higher, water falls further to

get to that spot, has accelerated more, so is get to that spot, has accelerated more, so is faster.faster.

– Stream cuts off meander loops, Stream cuts off meander loops, straightens, uses a smaller, lower, straightens, uses a smaller, lower, floodplainfloodplain

– Older, higher floodplain now a “terrace”Older, higher floodplain now a “terrace”

River meanders across floodplain

Base level drops,or region uplifts.Water goes faster, stream cuts down into channel, straightens, new lower floodplain narrower.

Old, unused floodplain a terrace

Further incision cuts a new terrace

Repeat

Depositional and Erosional TerracesDepositional and Erosional Terraces

Alluvial Stream

Bedrock Stream

DeltasDeltas

• Occur where stream hits slow waterOccur where stream hits slow water• Competence decreases to nearly zero, Competence decreases to nearly zero,

suspended load is dropped suspended load is dropped • Deposits build out into lake or sheltered sea, Deposits build out into lake or sheltered sea,

extending the length of the riverextending the length of the river

• Consists of three types of bedsConsists of three types of beds• Topset beds - horizontal, surface deltaTopset beds - horizontal, surface delta• Foreset beds - delta slopeForeset beds - delta slope• Bottomset beds – horizontal, bottom sea or lakeBottomset beds – horizontal, bottom sea or lake

Deltas have strata deposited in a characteristic geometryTopsets, Foresets and Bottomsets

Actually much more complex: many distributary channel deposits

The Mississippi River Delta changes location with time

General trend: Delta builds Seaward. If Floodwaters happen to reach

the sea via a shortcut, they flow faster inthe shorter (steeper) path, cutting a

new channel. The old channel is abandoned.

A fan-delta in a lakebirdsfoot

topsets

foresets

bottomsets

An An Alluvial FanAlluvial Fan in Death Valley in Death ValleySudden drop in competenceSudden drop in competence

Fast, steep bedrock stream hits flat plainFast, steep bedrock stream hits flat plain

Colluvial FansColluvial Fans

Bulk of stream load

Small Scale

Large scale