Geomorphology Logo| Home | Syllabus | Calendar | Other | Grades | Geog/Geol Dept | UWSP |

CHANNEL PATTERNS

photo: river valleyA. Introduction

Interaction of driving and resisting forces results in:

Learning Outcomes: you should be able to:

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


B. Single Strand Channels

Straight versus meandering

Characteristics

  • meandering thalweg

  • alternate point bars, pools, riffles, & cutbanks

  • meander growth & downstream migration

  • bar and swale topography

  • variable cross-channel profile

photo: point bar, cut bank USFWS
photo: M. LeFever. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/
diagram: meander development & features Alaska
photo: bar & swale topography
photo: U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

Pools and riffles

Meanders

  • meander development & growth

    • meandering thalweg alone insufficient to generate meandering channel

    • requires local, rather than widespread bank erosion

    • requires lateral erosion & deposition

    • requires helical flow transporting material from meander bend and depositing it in riffle or next point bar downstream

  • meander cutoffs

    • oxbow lakes

    • meander scars

  • balance between available energy & available sediment

    link icon: photo Boyce, LA 1970 link icon: photo Boyce, LA 1981 link icon: photo Boyce, LA 1994 link icon: photo Boyce, LA 2004 (Boyce, LA 1970, 1981, 1994 & 2004)

Wisconsin
photo: meandering river Wisconsin
photo: © Louis Maher. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ID hysfdg
Alaska
photo: meandering river Alaska
photo: S. Hillebrand. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

Step-pool sequences


C. Multi-strand Channels

Braided

  • characteristics

    • water strands divide around coarse-grained, unstable bars

    • frequent changes in size, location & number of bars

    • large total channel width 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 establishment of vegetation on bars

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

photo: 
      braided river
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 forms an enchannel ridge that diverts flow in two directions

    • both processes promoted by:

      • stable, cohesive banks that limit channel widening

      • one or more mechanisms that promote localized overbank flooding

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

D. Summary

Single strand: straight & meandering

  • meandering thalweg results in:

    • spatially variable erosion & deposition

    • formation of pools, riffles, cutbanks, & point bars

  • imbalances in available energy & available sediment cause meander growth/cutoff and associated adjustments in gradient

Multi-strand: braided & anabranching

  • widespread bank erosion may produce braiding

  • localized avulsion in cohesive sediment may produce anabranching channels

Temporal & spatial variability in driving/resisting forces causes spatial & temporal changes in channel pattern & position

  • changes in slope (gradient) due to tectonic activity or 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: straight river channel
photo: B. Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ID h27o4h

photo: straight river channel
photo: R. Hagerty. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/


| Next Lecture | Previous Lecture | Calendar | Home |
©Karen A. Lemke: klemke@uwsp.edu
Last revised September 9, 2014