| Summary: | Glacial Horse Lake (Wenatachee, Washington) was obliterated by the first Missoula flood that flowed through Wenatachee. Did this first Missoula flood change the environment and hydrology of Glacial Horse Lake and cause the demise or was the lake already in recession? To answer this question, we examined the glaciolacustrine deposits in detail in Horse Lake Canyon near Wenatchee, and qualitatively examined the varves in the deposit. Next, we used a grain size analyzer to obtain the mean grain size of each sample to distinguish laminae or beds from each stratigraphic layer. Then we statistically verified the results by using z-scores to quantify the variations of mean grain sizes between laminae or beds. At the end of the stratigraphic record the varves increase in thickness indicating increased sediment deposition in Glacial Horse Lake prior to the first Missoula flood. This increase in sediment accumulation could be the result of additional inflow of water from the Cordilleran Ice Sheet as it melts. Therefore, Glacial Horse Lake was likely already in recession prior to the first Missoula flood, and the first flood obliterated what was left of the lake.
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