High stand and catastrophic draining of intracaldera Surprise Lake, Aniakchak volcano, Alaska: A section in Geologic studies in Alaska by the U.S. Geological Survey, 1993

Wave-cut terraces and multiple exposures of lacustrine sediment indicate a former, more extensive stand of intracaldera Surprise Lake in the crater of Aniakchak volcano. The lake once covered nearly half of the caldera floor and had an estimated volume of about 3.7x10⁹ m³. A terrace that marks the high stand of the lake is traceable along the north caldera wall to a break in slope near the top of a v-shaped notch (The Gates) in the caldera rim. Downstream from The Gates, the Aniakchak River flows through a broad, terraced, and boulder-strewn valley. Results from our preliminary investigations suggest that Surprise Lake may have been at its high stand during the explosive destruction of an intracaldera stratocone sometime after 464 yr B.P. Stratigraphic relations suggest that the lake may have drained during this eruptive episode. We speculate that the eruptive activity caused water in the lake to overtop the caldera rim at The Gates, initiating failure of the caldera-rim dam and subsequent catastrophic drainage of Surprise Lake.