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., 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 VariableResponseSlope ↑ LoadAggradation↑ ↓ LoadDowncutting↓ ↑ DischargeDowncutting↓ ↓ DischargeAggradation↑

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

Sinuosity = stream length / valley length Meander wavelength: –Large streams have large meanders and vice versa Point bars – deposits on inside of meanders Cut banks – erosion of outside of meanders Oxbow 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