Anastomosed Fluvial Deposits: Modern Examples from Western Canada

  1. J. D. Collinson and
  2. J. Lewin
  1. Derald G. Smith

Published Online: 29 APR 2009

DOI: 10.1002/9781444303773.ch12

Modern and Ancient Fluvial Systems

Modern and Ancient Fluvial Systems

How to Cite

Smith, D. G. (1983) Anastomosed Fluvial Deposits: Modern Examples from Western Canada, in Modern and Ancient Fluvial Systems (eds J. D. Collinson and J. Lewin), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303773.ch12

Author Information

  1. Department of Geography, The University of Calgary, Calgary, Alberta T2N 1N4, Canada

Publication History

  1. Published Online: 29 APR 2009
  2. Published Print: 7 FEB 1983

ISBN Information

Print ISBN: 9780632009978

Online ISBN: 9781444303773

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Keywords:

  • anastomosed fluvial deposits - modern examples from Western Canada;
  • Upper Columbia River;
  • anastomosed sedimentary environments and sediments;
  • aggradation rates of anastomosed systems;
  • anastomosed facies association

Summary

Facies of two Canadian modern anastomosing river systems are discussed, the upper Columbia and lower Saskatchewan Rivers, which occur in intermontane and plains settings respectively. Both systems contain aggrading, multiple, low gradient, sand bed channels with adjacent splay, levée, and shallow wetland environments, all aggrading in accordance with channel sedimentation. While aggrading cross-valley alluvial fans or subsidence tend to control sedimentation rates in intermontane valleys, basin subsidence and/or regional tilting controls deposition rates in plains settings.

Sedimentation style in the upper Columbia River valley (120 × 1·5 km) consists of low-sinuosity, stable channels, depositing multi-storied channel sands and numerous sandy crevasse splay deposits. Channel deposits form as sand stringers laterally contained by deposits of levée silt and lacustrine mud. Aggrading at an average rate of 60 cm per 100 years over the past 2500 years, the anastomosing system is very dynamic, exhibiting avulsions and channel fills.

Deposition in the lower Saskatchewan River valley (120 × 80 km) a much wider basin with slower aggradation rates (29 cm 100 yr−1, C-14 date on peat buried beneath a levée) results in laterally extensive sheets of overbank levée deposits of fine sand which grade into even more laterally extensive thick deposits of peat. With time, some dominant channels become highly sinuous, thus causing increased flow resistance, major avulsions upriver and eventual channel filling and abandonment. Facies differences of the two anastomosed river systems are believed to be caused by both the rate of sedimentation and width of the sedimentary basin.