Geomorphic response of Lillooet River, British Columbia, to meander cutoffs and base level lowering

Abstract

A detailed record of channel profiles, slopes, and stream discharge on Lillooet River provides an opportunity to study the effects of natural and artificial channel changes that have occurred over the past century. We analyze the long-term effects of channel alterations that may affect flood hazard. In the mid 1940s several meanders were artificially severed, side channels blocked off, and the level of downstream Lillooet Lake was lowered. These measures were thought to increase hydrologic efficiency and decrease flood risk in the largely agricultural valley. Between 1947 and 1994 average channel width in the upper reaches decreased by 50%, most of which occurred by the late 1950s. Between 1945 and 1969 Lillooet River degraded its bed elevation by 3–4m (12.5 to 16.7cma−1) in the upper reaches and up to 2m (8.3cma−1) in the lower reaches. This sudden and profound degradation compares to average bed elevation increases of 2.4cma−1 prior to the engineering works. Between 1969 and 1985 the cross section area increased by 22% in the upper reaches and 13% in the lower reaches and decreased to 12% and 8%, respectively, for the time period 1985 to 2000. The increased sediment supply that was caused by channel straightening accelerated delta advance in Lillooet Lake from 7ma−1 (1858 to 1948) to 30ma−1 for the five-year period following the 1948 channel works. These rates have decreased over time, but with a current advance rate of 10.5ma−1 (1986–2009) are still above the long-term average prior to the channel changes. This study demonstrates the time scale, direction, and magnitude of channel changes following significant artificial river alterations. While the initial goal of decreasing flood risk had been achieved in the short term, the lower river apparently is slowly returning to an overall aggradational phase. Ongoing delta advance will ultimately increase channel elevations in the lower reaches and lead to significant flood hazards for populated areas.