Fluvial Processes

Connecticut River, Amherst, MA. Holyoke Range in distance. Foto: Lachniet (2003)

Flow velocity profiles• Friction between water and sediment creates variable

velocity• And turbulence• Fastest in middle• Uppermost water slower – why?• “Average” velocity at 0.6 of depth

From Ritter et al., 2002. Process Geomorphology, Fourth Edition

Figure 6-4

Sediment Transport• Suspended Load

– Within the water column– Supported by turbulence– Typically silt and smaller

• Bedload– Typically silt and larger– Traction: Rolling and sliding– Saltation: bouncing

Suspended Load River

“Encontro das Aguas” – Juncture of Rios Amazonas and Negro, Brazil. Foto: Lachniet (1994)

Copyright © Richard Kesel 2002

Suspended load and Bed load River

Entrainment• Process by which sediment starts moving• Competence

– largest particle size entrained for given hydraulic conditions

– Depends on • Mean stream velocity (Hjulstrom diagram)• Turbulence• Grain shape and packing

Hjulstrom Diagram

“entrainment”

Sediment load and work• Corrasion

– Erosion of bank by water• Abrasion:

– Erosion by entrained sediment– More efficient at eroding than water– Produces bank failures

• Plucking: – Entrainment of bedrock blocks from channel floor– Particularly effective in jointed rocks

• Abrasion + Plucking– Creates potholes– Erodes bedrock channels

Bedrock Channel

River channel in Basalt, Pacific Slope of Panama. Foto: Lachniet (2002)

Potholes

Potholes in Foz do Iguaçu, Parana River, Brazil, Lachniet 2010

Deposition• Sediment deposition occurs when

– ↓ Velocity– ↓ Gradient– ↑ Sediment load

• Sediment erosion and deposition occurs within and along the channel, resulting in scour and fill– If net deposition, the river is aggrading– If net erosion, the river is downcutting

Quasi-Equilibrium conditions• Hydraulic variables adjust to changes in sediment load and

discharge• A change in any variable requires a change in the other variables

Variable Response Slope

↑ Load Aggradation ↑

↓ Load Downcutting ↓

↑ Discharge Downcutting ↓

↓ Discharge Aggradation ↑

Graded Stream Profile

• Represents an equilibrium stream profile

• Decreasing slopes with distance• “Graded” to base level • Base level is the lowest altitude

along river– Usually the ocean or a lake

• Knickpoints are interruptions to graded stream driven by geologic structure or a change in base level Knick Points

Mother of all Knickpoints

Foz do Iguaҫu, Brazil

Channel Patterns

• Straight, meandering, and braided are three main types

• Subdivided by type of load transported

• Thalweg: deepest part of the channel

Figure 6-25

Processes in straight and meandering channels

• Alternate bars• Pools• Point bars• Pools and riffles

Copyright © Keith Richards 2002

Even Straight channels (in this case artificial) end up meandering

Note the Alternate Bars and Pools

Meandering Rivers

Meandering Rivers

• Sinuosity = stream length / valley length• Meander wavelength:

– Large streams have large meanders and vice versa

Point bars – deposits on inside of meandersCut banks – erosion of outside of meandersOxbow Lake – prodcued where a meander cutoff

becomes a lake

Meandering Rivers

• Initial obstruction shifts current

• Positive feedback accentuates meanders

• Helical flow pattern

Meander Cutoff

Meander loop cutoff. Katalla Valley, Alaska. Foto: Lachniet (2000)

Oxbow Lake

Oxbow Lake. Katalla Valley, Alaska. Foto: Lachniet (2000)

Braided Rivers

• Most common in glacial and arid environments• Channel is wide but shallow

– 1) Erodible Banks• Usually coarse grained

– 2) Abundant sediment load• Load is greater than can be removed

– 3) Rapid and frequent discharge variations• High Q = bank erosion and increases load• Low Q = sediment deposited in braid bars

Braided River, Yakataga, Alaska

Near Yakataga, Alaska. Foto: Lachniet (2000)

• Braided main channel, and braided patterns on alluvial fan, Death Valley, CA

Anastomosing Channels– Aka “anabranching”– Low gradients– Low width/depth ratios– Stable vegetated islands– Fine grained bank

sediments– Interconnected networks

of channels

Anastomosing River, N. Nevada. Lachniet, 2005