Morphodynamics of an extended bar complex, Fraser River, British Columbia

Abstract

ABSTRACTWe investigate large‐scale morphologic changes over a 65‐year period in a major sedimentation zone within the 50‐km long wandering gravel‐bed reach of lower Fraser River, British Columbia. This reach remains in a relatively pristine state compared with many major rivers in populated regions even though various forms of human interference have occurred over the past century to mitigate flooding and erosion concerns. A template of channel evolutionary development is identified, consisting of extended periods of orderly accretion of unit bars by deposition of gravel sheets, controlled by the major riffles in the river. When this process constricts channel conveyance an avulsion occurs and the major bar complexes are reorganized. The recognition of fairly predictable short‐term behavior can potentially aid the development of models of sediment transport and channel evolution. Accordingly, results of the detailed descriptive analysis are compared with a GIS‐based probabilistic approach developed from transition‐state analysis. This reveals that the river has a far stronger tendency to retain its existing morphology than to change its form within a roughly decennial time scale – regardless of the magnitude of intervening flows – but that this tendency gradually declines over time as major bar and island complexes are re‐worked by sediment exchange. The timescale for major bar development appears to be of the order 30 years, while complete sediment exchange occurs within a century. Copyright © 2012 John Wiley & Sons, Ltd.