Evidence for Late Pleistocene Megafloods in Southcentral Alaska, USA

Reanalysis of geomorphic features of the lower Matanuska Valley in Southcentral Alaska, U.S.A., previously described as crevasse-fill-ridge complexes or as the result of compressive forces within the stagnant Matanuska glacier generated by the impinging resurgent Knik glacier, leads us to reinterpret them as a giant current ripple train. The train has a cord length of 0.9 km and ripple heights up to 20 m. The most likely sources for floodwaters sufficient to create ripples of this scale were glacial lakes Atna and Susitna. These Pleistocene lakes repeatedly formed, and may have merged, within the Copper River Basin, a broad, flat intermontane plateau surrounded by the Alaska Range, and the Chugach, Talkeetna, and Wrangell mountains. At least five times during the Late Pleistocene, glaciers from the surrounding mountains dammed outlet rivers, forming large dynamic proglacial lakes. Glacial Lake Atna/Susitna had a reported maximum surface elevation of 975 m. The modern elevation of Tahneta Pass, the divide between the Copper River Basin and the Matanuska River, is 900 m. We estimate the glacial lake volume available to decant through Tahneta Pass was in the range of 4 x 10² –1.6 x 10³ km³. Evidence from other, lower outlet passes suggests multiple floods incrementally drained the paleolake.