SEDIMENTATION IN CHILKO LAKE - A RECORD OF THE GEOMORPHIC ENVIRONMENTOF THE EASTERN COAST MOUNTAINS OF BRITISH-COLUMBIA, CANADA

Chilko Lake is the largest and deepest lake draining the Coast Mountai ns of British Columbia. A sedimentary record from the 60-km long lake was constructed by using 3.5-Hz subbottom acoustic profiles and short cores of surface sediments. Sediment accumulation since deglaciation i s remarkably thin, averaging less than 10 m, except in the central dee p part of the lake, despite extensive activity of Holocene and modem g laciers in the 1960-km(2) contributing watershed. Sediment focusing of silts and clays in the main part of the lake occurs mainly by overflo w/interflow processes from the Edmond and Nine Mile/Deschamps drainage s. Sandy turbidites from these drainages and other smaller streams, ho wever, extend for several kilometers into the lake basin. Distinct lam inae in short cores from the central lake suggest that modern rates of deposition reach a maximum of 2.2 mm a(-1). Pb-210 dating and inferen ces from the acoustic results, however, indicate a lake-wide, long-ter m average rate of less than 1 mm a(-1), which is equivalent to a speci fic sediment yield of about 80 t km(-2) a(-1). This specific yield is amongst the lowest for glacier-fed lakes of the southern Canadian Cord illera. A low sediment supply can be related to three factors: (1) a m uch drier climate on the leeward side of the Coast Mountains producing lower specific discharges; (2) fewer autumn rainstorms that affect ma inly lakes west of the Coast Mountains and (3) opportunities for stora ge of sediment in the lower 20 km of Edmond River, which drains the mo st heavily glacierized portion of the watershed. (C) 1998 Elsevier Sci ence B.V. All rights reserved.