RECENT HOLOCENE CHANGES IN SEDIMENTATION IN A LANDSLIDE-DAMMED LAKE IN THE CASCADE-MOUNTAINS, SOUTHWESTERN BRITISH-COLUMBIA, CANADA

Landslide-damming of Silver Lake in the Cascade Mountains took place a bout 1000BP, Delta growth favoured the faulted west valley side. At th e commencement of logging activity in AD 1946 the course of Silverhope Creek was diverted away from access roads. As a consequence, the chan nel shifted from the west to the east side of the valley, forming a ne w delta. Six gravity lake cores and SCUBA surveys of lake sediments re veal changes in the depositional regime. In two cores, late Pleistocen e-early Holocene sediments are preserved. Old delta abandonment, new d elta growth, reorientation of turbidity channels and confluence zones are discussed. Cs-137 data show that, Following channel diversion, lit tle or no further deposition occurred on the old delta. Assuming that the 1964 Cs-137 maximum has been recorded in each curt, sedimentation rates apparently decrease to the NNW, away from the new delta, and Cs- 137 values increase northwards. Stratigraphy, C-14 and Cs-137 dates in dicate that there has been a recent increase in sedimentation rate of about 20 rimes near the deltas, to about 10 times in a mid-lake locati on. Erosion and the release of destabilized glaciolacustrine valley-si de sediments into the creek enhances sedimentation in Silver Lake. Uns table sediments on steep valley sides prohibit forest regrowth.