GLACIAL LAKE INVERMERE, UPPER COLUMBIA RIVER VALLEY, BRITISH-COLUMBIA - A PALEOGEOGRAPHIC RECONSTRUCTION

Terraces of thick lacustrine silt and deltaic gravel flank parts of the valley floor of the Rocky Mountain Trench between Skookumchuck and Donald, British Columbia. These indicate the presence of former Late Wisconsinan glacial Lake Invermere, which at its maximum extent occupied the Rocky Mountain Trench from Bluewater Creek, 6 km north of Donald, to 7 km north of Skookumchuck. The lake was 210 km long, an average of 2.5 km wide by 100 m deep, and had an area of 530 km2. Retreating glacier ice is interpreted to have formed a dam at the northern end of the lake, and blockage to the south resulted from a sediment valley fill. Glacial Lake Invermere formed as two water bodies, at elevations 885 and 900 m asl, separated by glacier ice. These two water bodies later joined to form a continuous lake at 835 m asl. Evidence of isostatic tilting is absent, suggesting uniform ice thickness and thinning, a pattern contrary to that inferred for other areas of southern British Columbia. After breaching of the valley fill at its south end, the lake terminated with final melting of Rocky Mountain Trench ice. At that time the southerly flow of water reversed to a northerly direction. A radiocarbon date from an adjacent valley indicates the lake drained prior to 10 000 BP.