Recent Holocene changes in sedimentation in a landslide-dammed lake in the Cascade Mountains, southwestern British Columbia, Canada

Abstract

Landslide-damming of Silver Lake in the Cascade Mountains took place about 1000BP. Delta growth favoured the faulted west valley side. At the commencement of logging activity in AD 1946 the course of Silverhope Creek was diverted away from access roads. As a consequence, the channel shifted from the west to the east side of the valley, forming a new delta. Six gravity lake cores and SCUBA surveys of lake sediments reveal changes in the depositional regime. In two cores, late Pleistocene-early Holocene sediments are pre served. Old delta abandonment, new delta growth, reorientation of turbidity channels and confluence zones are discussed. 137Cs data show that, following channel diversion, little or no further deposition occurred on the old delta. Assuming that the 1964 137Cs maximum has been recorded in each core, sedimentation rates appar ently decrease to the NNW, away from the new delta, and 137Cs values increase northwards. Stratigraphy, 14C and 137Cs dates indicate that there has been a recent increase in sedimentation rate of about 20 times near the deltas, to about 10 times in a mid-lake location. Erosion and the release of destabilized glaciolacustrine valley- side sediments into the creek enhances sedimentation in Silver Lake. Unstable sediments on steep valley sides prohibit forest regrowth.