Streams & Transportation

• Streams are major erosive force
• Topics:

– How water flows in currents
– How currents carry sediment
– How streams erode rock
– How streams develop specific drainage patterns

• Stream – any flowing body of water (includes rivers)
• River – very large streams

• Importance of streams

– Agriculture
– Transportation
– Erosion
– Deposition

Stream flow

• Fluid flow & streamlines

– Laminar flow: all stream lines parallel

• Generally slow flow

– Turbulent flow: streamlines cross each other

• Generally fast flow
• Eddies common
• May be high or low turbulence
 
 

• Turbulent vs. Laminar

– Depends on speed
– Depends on geometry

• Mostly depth

– Depends on viscosity

• Resistance to flow
• Comes from attractive forces between fluid molecules
• Heat changes viscosity
• Low viscosity fluids more prone to turbulent flow

• Most streams are turbulent

– Fast flowing
– Water is low viscosity

• Near shore with slow flow may be laminar
• Ground water usually laminar

Stream Loads (Sediment)

• How is sediment carried by streams?
• Two types of sediment transportation:

– Suspended sediment: material temporarily or permanently suspended in the flow
– Bed load: material moved along bottom by sliding or rolling

• Faster currents carry larger particles
• Competence: the ability of a stream to carry particles of a given size
• Capacity: the total amount of sediment carried by a stream

• Capacity and competence are not necessarily related

– Large slow rivers (Mississippi) have high capacity but low competence

• Lots of stuff, but small grain size

– Mountain streams have low capacity by high competence

• Little stuff, but big grains (boulders)

Deposition

• How does material settle from suspension?

– Upward force of turbulence VS. downward force of gravity
– Settling velocity: the speed grains settle from suspension

• Grain size factor in how long grains stay suspended

– Large grains short time
– Small grain long time

• Saltation

– transportation of grains intermittently in suspension
– Common for sand grains

• Saltation leads to “bedforms”

– Commonly dunes and ripples
– Dunes large, up to meters high
– Ripples small, centimeters high
– Both have long direction perpendicular to current
– Form in similar way as air dunes

•
• Dunes migrate downstream
• Speed much slower than current
• Higher speed forms ripples on backs of dunes

– Ripples migrate

• At very high velocity all bedforms wiped out

– Rapid “sheet flow” along bottom of stream

• General summary:

– Clay size (small): suspension
– Sand size: saltation
– Course sand & gravel: bed load

• What is relationship between transportation mechanism and velocity?

• Hjulstrom Diagram

– Plot of grain size vs. current velocity

Erosion by streams

• Streams can erode consolidated material (rock)

– Abrasion
– Chemical and physical weathering
– Undercutting at waterfalls

Geomorphology of streams

• Valleys
• Channels

– Meanders & braided streams

• Flood plains
 

• Erosion leads to valleys

– Commonly V shaped, others have flat floors
– Depends on the amount of sediment

• Mountain valleys steep walled, erosive
• Lower stretches of river are flat floored, depositional
 

• River channels

– lowest part of the valley
– Carries the river at normal flow (called base flow)
– Varies between straight and meandering

 

• Meanders

– Most flows have meanders: gulf stream, lava flows, jet stream
– Current meanders within channel

• Outside of bends highest flow rate
• Inside of bends lowest flow rate

– Outside bend erosive: “cut bank”
– Inside bend depositional: “point bar”

• Meanders migrate through time

– Bends loop back and cut off
– Create “oxbow” lakes

• Meanders tend to mediate floods

– Reduce energy of flood

• Braided streams

– Places where streams split and rejoin in many channels
– Common where variations in flow and large amount of sediment

• E.g. at edge of glacier

• Floodplains

– Areas outside of river channel
– During flooding water spreads across floodplain
– Velocity decreases as water flows over large area

 

• Levees

– Slow velocity means low capacity
– Competence decreases
– Large material dropped near channel
– Fine material far from channel
– Elevated sides need channel called levees
– Fine grained material very fertile

Changes in distance and time

Discharge

• The volume of water carried past a particular point of a stream

– E.g. vol/time, ft3/sec

• Measurement

– area of channel x velocity of water

• Increase in discharge

– Area increase
– Velocity increase
– Both increase
– Velocity may decrease downstream because of lower slope

 

• Discharge tends to increase downstream

– Tributaries add water
– Not always the case, e.g. Suwannee river

Floods

• Times when rivers flow outside their channels
• Floods described with a certain probability of happening
• Probability corresponds to length of time that the flood will happen
• Recurrence interval
 

• Example

– 5 year flood
– Corresponds to a particular height of flood
– Probability of occurring every five years
– Determined based on empirical evidence

• Fairly reliable now, much stream gauging data
• Need to generate data for every river

Longitudinal Profile

• Dynamic equilibrium of erosion and deposition along length of stream
• Slope of river

– Plot of elevation versus distance along channel
– Called longitudinal profile
– Always concave upward curve

• Control of profile at lower end is base level

– End of stream when it flows to standing body of water

• Base levels may change

– Sedimentation decreases gradient
– Sealevel changes
– Building lakes create local base level

• Shallow gradient upstream – deposition
• Steep gradient downstream – erosion
 
 

• Alluvial fans

– Sediment deposited along mountain fronts
– Streams leave steep mountain areas into flat valleys

• Capacity and competence decrease
• Deposit sediment
• Commonly course at top and fine at bottom
 

• Terraces

– Uplift following formation of flood plain
– River erodes into flood plain
– Leaves old flood plain as flat areas on each side of channel

• Drainage divide: Topographic high between rivers

– Precipitation flows down one side or other

• Continental divide

– Separates water flowing to Pacific from water flowing to Atlantic

• Drainage basin: area of land surrounded by divides

– Funnels all streams within basins to common discharge point
– Variety of scales
– Basins within basins

 

• Streams erode back into basins
• Stream piracy

– One stream may breach divide
– Capture the discharge of part of stream in another basin

• Drainage patterns

– Shape of the tributaries controlled by geology
– Many different types

• Dendritic
• Rectangular
• Trellis
• radial
 

• Geologic history

– Relationship between the geologic history and drainage of an area

• Antecedent stream

– Stream erodes through ridge as uplifted

• Superposed stream

– Stream course controlled pattern developed in upper beds
– May cut across topography like antecedent stream

Delta

• Deposits of stream sediment at mouth

– Formed by lower current velocity as stream reaches base level
– Make topset, foreset, and bottomset beds

• Channels branch downstream at mouth

– Form distributaries

• Deltas grow seaward as sediment deposited
• Example

– Mississippi delta grown 1600 km in 150 my
– Originally in southern Illinois
 

• Controls on delta growth

– Tides, waves, rivers
– E.g. Mississippi delta grow outward because tides and waves small in Gulf