Rate of floodplain reworking in response to increasing storm-induced floods, Squamish River, south-western British Columbia, Canada



Relations among hydroclimatic and channel planform changes on Squamish River are presented for the period 1956–2007. Squamish River basin occupies 3600 km2 of mountainous terrain in south-western British Columbia, about 50 km north of Vancouver. The magnitude, volume and duration of extreme floods (Q ≥ 1500 m3/s) exhibit respective temporal increases of 50, 450 and 300%. The increase in extreme floods is attributed to the intensification of late-season (August–December) Pacific storms that have produced increases in precipitation amounts, intensity and duration of respectively 340, 200 and 200% over the same period. Changes in floodplain-surface area calculated from the geographic information system (GIS) differencing of sequential large-scale aerial photographs indicate that the rate of geomorphic change in Squamish River has accelerated during the 1980s to the mid-1990s. Among four study reaches of varying planform, erosional, depositional and cumulative changes in floodplain surface-area have rapidly increased. Channel-change activity after 1980 has increased by a factor of two to six compared with the period prior to 1980. Erosion is currently outpacing deposition in the majority of study reaches. Although channel geometry generally exhibits no uniform pattern of response to the increase in extreme floods, the meandering reaches have straightened over the duration of the study period. The increase in the magnitude and duration of the annual flood appears to be the principal cause of this recent acceleration of channel change. Copyright © 2010 John Wiley & Sons, Ltd.