| This study investigates the relation between sediment transport and channel deformation on a 70 km long wandering reach of lower Fraser River, British Columbia. The reach is characterized by an irregularly sinuous single-thread channel split around large bar and island complexes. An extensive network of secondary channels, produced from channel shifting and abandonment, is found within these complexes and along the adjacent floodplain. Most material accumulates within wide 'sedimentation zones' where deposited sand and gravel create an obstruction to downstream flow conveyance, and in areas of flow expansion where shear stress declines. These sites correspond to the location of prominent diagonal riffles. Changes to reach morphology are dominated by the transfer of coarse alluvial sediments. Small gravel sheets are attached to existing lateral and mid-channel bars, forcing compensating erosion across the channel and propagating instability downstream. Over periods of several decades, entire bars and bar/island complexes migrate downstream in association with the riffles.; Sediment budgets constructed for several periods between 1952 and 2003 show that the transport rate into the reach is highly variable over time and does not correlate well with the magnitude or frequency of large floods. Influx into the reach has apparently increased over time, from 180,000 m3 /yr from 1952 to 1984 to 494,000 m3/yr from 1999 to 2003.; Although the gravel reach remains a relatively pristine environment relative to most large navigable rivers in populated regions, the isolation of several large sloughs and backchannels, lateral confinement through bank hardening, and removal of sand and gravel from the channel have cumulatively reduced the complexity of channel morphology and reduced the channel gradient. Despite evidence that recent morphologic evolution is trending towards a more stable single-thread meandering habit, considerable downstream instability is expected to continue for several decades as this pattern continues to develop. The relations between sediment transport and channel deformation developed in this thesis should provide better tools to facilitate sound riverine management during this period, but additional survey is required to evaluate such change. (Abstract shortened by UMI.)... |
