Annual bed‐elevation regime in the alluvial channel of Squamish River,Southwestern British Columbia,Canada

The aim of this study is to examine the annual regime of channel scour and fill by monitoring bed‐elevation changes in a reach of Squamish River in southwestern British Columbia, Canada. Sonar surveys of 13 river cross‐sections in a sandy gravel‐bed single‐channel study reach were repeated biweekly over a full hydrologic year (1995/6). The survey results show that bedload movement occurs as waves or pulses forming bedwaves that appear to maintain an overall coherence with movement downstream. These bedwaves propagate downstream by a mode here termed pulse scour and pulse fill, a process distinguished from the conventional mode of scour and fill commonly associated with flood events (here termed local scour and local fill). Bedwave celerity was estimated to be about 15·5 m d⁻¹ corresponding to a bedwave residence time in the study reach of almost one hydrologic year. The total amount of local bed‐elevation change ranged between 0·22 m and 2·41 m during the period of study. Analysis of the bed‐elevation and flow data reveals that, because of the bedwave phenomenon, there is no simple relation between the mean bed‐elevation and discharge nor any strong linear correlation among cross‐sectional behaviour. The bed‐elevation data also suggest that complex changes to the bed within a cross‐section are masked when the bed is viewed in one dimension, although no definitive trends in bed behaviour were found in the two‐dimensional analysis. Although a weak seasonal effect is evident in this study, the bed‐elevation regime is dominated by sediment supply‐driven fluctuations in bedload transport occurring at timescales shorter than the seasonal fluctuation in discharge. The study also indicates that bed‐elevation monitoring on Squamish River, and others like it, for purposes of detecting and measuring aggradation/degradation must take into account very considerable and normal channel‐bed variability operating at timescales from hours to months. Copyright © 2000 John Wiley & Sons, Ltd.