Intensifying storms, floods and channel change: Squamish River, BC (1956-2007).

This study examines relations among hydroclimatic and channel changes on Squamish River in southwestern British Columbia (1956-2007). Magnitude, volume and duration of extreme floods (Q≥1500 m3/s) exhibit a 50, 450 and 300 percent respective increase with time and the annual-flood series is non-stationary with the largest floods being more recent. The increase in extreme floods is attributed to the intensification of late-season (Aug-Dec) Pacific storms that have produced increases in precipitation amounts, intensity and duration of respectively 340, 200 and 200 percent over the same period. Changes in floodplain-surface area, calculated from GIS differencing of sequential large-scale aerial photographs, indicate that the rate of geomorphic change in Squamish River has accelerated between 1956-2007, a result of an increase in the magnitude and duration of the annual flood. Channel-change activity after 1980 has increased by a factor of 2 to 6 compared with the period prior to 1980.