THE LATE QUATERNARY SEDIMENTARY RECORD OF STAVE LAKE, SOUTHWESTERN BRITISH-COLUMBIA

The postglacial sedimentary record of 59 km2 Stave Lake was investigated using 3.5 kHz subbottom profiles and cores from the sediment surface. The acoustic data show a thin cover of acoustically transparent sediment (unit 1) overlying bedrock or glacial sediment on the floor of the lake. Overlying acoustically stratified sediment is divided into unit 2, which thins from 28 m in the south of the lake to less than 12 m in the north, and unit 3, which thins from 20 m in the north to about 12 m in the south. Unit 1 is interpreted as resulting from deposition in a relatively quiet lacustrine or marine environment following retreat of Vashon glaciers about 13 ka ago. Unit 2 is ascribed to deposits of sediment in runoff to Stave Lake from the Sumas ice sheet in the Fraser Valley and connecting valleys to the Stave Basin. Following the retreat of the Sumas ice about 11 ka ago, deposition of unit 3 resulted almost entirely from sediment input from Stave River entering at the north end of the lake. Raising the lake in 1912 by damming for hydroelectric generation resulted in deposition of a thin but distinct marker horizon in the sediment, from which modern rates of accumulation averaging 3 mm/a are estimated. These are more than twice the average rates for the Holocene estimated from the total thickness of unit 3. The rates of sediment yield calculated from accumulation in Stave Lake are 4.5 x 10(5) kg . km-2 . a-1 (modern) and less than 2 x 10(5) kg . km-2 . a-1 (averaged over the Holocene).