Abstract

Late Pleistocene glaciers dammed Glacial Lake Champagne in the valley of Dezadeash River between a westward-flowing glacier in the Takhini valley and eastward-flowing glaciers from the St. Elias complex. Modern Kusawa Lake lies in the southern extension of Lake Champagne. Geophysical and geomorphic evidence documents the deglaciation of the lake, the presence of Lake Champagne, and the postglacial sedimentary environment of the basin. During the main phase of Lake Champagne, the water level stood at 772 m in the northern part of Kusawa Lake and 756 m in northern Dezadeash valley, both probably controlled by a spillway floored at 756 m to the north into the Nordenskiold River. This indicates differential isostatic rebound of 0.2 m/km from south to north. At that time a trunk glacier occupied the southern portion of Kusawa Lake, depositing a thick sequence of sediment in the basin. A glacier in the Primrose valley and the Takhini trunk glacier built large deltas into Lake Champagne. Subsequently, the level fell to 744 m, controlled by a spillway around the sediment plug at the outlet of Kusawa Lake, and the trunk glacier retreated from Kusawa Lake. Lacustrine sediment washed from the now substantially exposed valley sides was deposited as a distinctive facies in the north-central portion of Kusawa Lake. Incision of the delta at the outlet of the lake lowered its level to a major strandline at 714 m and eventually to its present level of 671 m. In the southern portion of the lake, a single sedimentary facies documents continuous glacilacustrine deposition from sediment originating in tributary basins still containing 11%–14% glacier cover.

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