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A. Introduction

• Driving force: hydraulic conditions

• Resisting force: sediment characteristics of channel bed & banks

• Agenda - examine:

  • stream channel patterns & their characteristics

  • processes of entrainment, transportation & deposition that form & maintain those channel patterns

photo: T.A. Blake. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

B. Single Strand Channels

Straight versus meandering

• sinuosity: stream channel length divided by length of meander belt axis or by valley length

  photo: S. Hillebrand. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

• sinuosity of 1.5 is dividing point between straight & meandering

  photo: A. Copley © Oklahoma University. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ID hn86ji

  photo: S. Hillebrand. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

Characteristics meandering thalweg alternate point bars, pools, riffles, & cutbanks meander growth & downstream migration bar and swale topography variable cross-channel profile

photo: M. LeFever. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

Alaska photo: U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

Pools and riffles

• characteristics at low discharges

  RIFFLES	POOLS
  shallow (& wide)	deep (& narrow)
  high velocity	low velocity
  steep water surface gradient	gentle water surface gradient
  coarse grained bed material	fine grained bed material
  divergent flow (facilitates deposition)	convergent flow (facilitates scour)

• secondary flow patterns: spiral or helical flow

  • outside of bends - water surface is elevated, which generates an accelerating downward motion, which scours the bank

  • inside of bends - flow is decelerating and moving upwards, which results in deposition of material

• formation

  • velocity reversal hypothesis: pools & riffles form when discharge exceeds a threshold such that pool velocity exceeds riffle velocity

  • as discharge increases, rate of change in:

    • depth is greater in riffles than in pools

    • velocity is less in riffles than in pools

    • water surface slope is less in riffles than in pools

    • result: as flow increases, pools & riffles become more similar

  • above a threshold discharge, critical bed velocity & bed shear stress are greater in pools than in riffles

Step-pool sequences

• gravel bed rivers with gradients steeper than 2-3%

• steps: accumulations of woody debris, bedrock or interlocking cobbles & boulders; high gradient & velocity

• pools: fine grained bed material; low gradient & slow flow

C. Multi-strand Channels

Braided characteristics water strands divide around coarse-grained, unstable bars frequent changes in size, location & number of bars total channel width is large compared to channel depth gradient generally steeper than meandering rivers factors associated with braiding: easily eroded banks - widespread bank erosion abundant bed load rapid & frequent variations in discharge disallows vegetation to establish on bars

photo: M. Emery. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/ photo: K.A. Lemke

Anabranching relatively permanent system of multiple sinuous channels with cohesive banks formation avulsion: local occurrence of overbank flow cuts new channel into existing floodplain deposition results in formation of enchannel ridge that diverts flow into 2 directions both processes promoted by: stable, cohesive banks that limit channel widening one or more mechanisms that promote localized overbank flooding

photo: H.J.A. Berendsen http://www.geo.uu.nl/fg/palaeogeography/results/fluvialstyle photo: © Michael Collier. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ixvt9i

D. Summary

Single strand: straight & braided meandering thalweg results in: spatially variable erosion & deposition formation of pools, riffles, cutbanks, & point bars imbalances in available energy & available sediment results in growth or cutoff of meanders Multi-strand: braided & anabranching widespread bank erosion may produce braiding localized avulsion in cohesive sediment may produce anabranching channels Temporal and spatial variability in driving and resisting forces result in spatial and temporal changes in channel pattern and position changes in slope (gradient) due to tectonic activity, changes in base level changes in available energy due to climatic change or contribution of tributary streams changes in resistance of banks to erosion changes in sediment supplied to stream due to land use change, glacials - interglacials

photo: B. Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ID h27o4h photo: R. Hagerty. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/