CHANNEL PATTERNS

A. Introduction

• location of erosion & deposition (bed/banks) affects channel shape

• channel pattern: planform of channel; shape as seen from a bird's eye view

• classification: straight, meandering, braided, anabranching

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

  • sinuosity of 1.5 is dividing point between straight & meandering

• what are the defining characteristics of these four channel patterns?

• under what discharge and load conditions do these channels form?

B. Straight Channels

1. Occurrence

2. Characteristics

• sinuosity < 1.5

• thalweg: line of maximum velocity; meanders back and forth between banks

• may get alternate bars, pools & riffles

3. Pools and riffles

a. 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

b. formation

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

• secondary flow patterns: convergent & divergent flow

  • convergent flow in pools facilitates scour

  • divergent flow in riffles facilitates deposition

4. 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

photo: US Fish & Wildlife Service http://dls.fws.gov/DATA/files/70D3C02C-65B8-D693-7EEA8E5CB3662D68.jpg

C. Meandering Channels

1. Occurrence

2. Characteristics

• sinuosity > 1.5

• ratios among meander wavelength, amplitude, curvature & width fairly constant

• meandering thalweg, pools, riffles, point bars

3. 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

4. Meander development

• change from straight to meandering

  • meandering thalweg alone is insufficient to generate a meandering channel

  • requires local, rather than widespread bank erosion

  • required sediment transport; helical flow transporting material from meander bend and depositing it in riffle or next point bar downstream

• meander growth & cutoff
  photo: Copyright © Louis Maher; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ID hysfcl
  

D. Braided Channels

1. Characteristics

• wide, shallow, straight channel with numerous strands of water

• water strands divide around coarse-grained bars

• frequent changes in location & number of water strands

• total channel width is large compared to channel depth

• gradient generally steeper than meandering rivers

2. Factors associated with braiding

• easily eroded banks - widespread bank erosion

• abundant bed material/bed load; often large sized

• rapid & frequent variations in discharge disallows vegetation to establish on bars

photo: H.J.A. Berendsen http://www.geo.uu.nl/fg/palaeogeography/results/fluvialstyle

E. Anabranching Channels

1. Characteristics

• relatively permanent system of multiple sinuous channels with cohesive banks

2. 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

F. Summary

1. Four basic channel patterns form a continuum

2. Conditions affecting formation

a. balance between hydraulic characteristics & sediment load affects channel planform and vice versa

b. threshold values for controlling variables

• threshold between meandering & braided: increase slope at a given Q or increase Q at a given slope and pattern will change from meandering to braided

c. ability to do work changes over time and space

• system maintains equilibrium by

  • eroding material when excess energy is available - degradation or lateral migration

  • depositing material when insufficient energy is available - aggradation or lateral deposition

• result: channel shape & position may change

3. Changes in channel pattern may be associated with:

• 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