MILLENNIAL-SCALE PATTERNS OF STREAMFLOW VARIABILITY INFERRED FROM ANNUALLY LAMINATED LAKE SEDIMENTS, COAST MOUNTAINS, CANADA

We recovered five percussion cores ranging in length from 2.1 to 3.5 m from Cheakamus Lake (50^oN, 123^oW), British Columbia, to determine if the lake’s annually laminated (varved) sediments contain proxy information on changes in the hydrologic regime during the late Holocene. Cheakamus Lake is an oligotrophic lake draining a moderately (25 percent) glacierized watershed in the southern Coast Mountain, approximately 100 km north of Vancouver.  Recovered sediments are inorganic, finely laminated, clayey silts. The moderate correlation (r=0.75; n=41) between laminae thickness and annual flood magnitude obtained from a nearby gauging station suggests that the laminae represent clastic varves. Their thickness (2.97 ± 2.6mm; n=947) and excellent preservation allowed us to subdivide the varves into six major types. Comparison between varve type and annual hydrographs over the period of instrumentation allows us to attribute the six varve types to (a) snowmelt flooding, (b) autumn flooding, (c) two major inflow events, (d) three major inflow events, (e) more than three major inflow events, and (f) periods of sustained glacial runoff. Varves attributed to glacial runoff (type f) and late autumn floods (type b) are dominant. Markov chain analysis indicates that varve types (b) and (f) exhibit serial dependence and are most common at ca. AD1300-1320, AD1380-1410, AD1470-1500, AD1710-1730, AD1880-1906, and AD1916-1945. From Northern Hemispheric reconstructions of air temperatures, we conclude that these periods coincide with decades that are warmer than the long-term average. In contrast, varves interpreted to form predominantly during early season floods appear to be randomly distributed through time. These results confirm the importance of sediment production and transfers during the summer and autumn runoff season in the Coast Mountains over the past 1000 years.