Pleistocene Glacial Lake Ahtna, Alaska: Constraints on Lake Extent and Volume, and Relationships to Modern Glaciers

It is postulated that the Copper River Basin (CRB) in south central Alaska contained the large glacier Lake Ahtna during the late Pleistocene [1,2]. Evidence for the existence of the paleo lake includes interpreted lake induced wavecut or depositional benches and strandlines, thick sequences of rhythmite and varve deposits, extensive diamicton and glaciolacustrine deposits, and kames and kettles. The widespread distribution of these features suggests that the glacial Lake Ahtna at one time, or through time, evolved piecemeal to cover >9000km2. Outstanding questions include: what was the evolution of lake extent and volume through the late Pleistocene; what portions of the lake volume contained water and ice; where, and how high were the ice dams that contained the lake; where were the lake spillways; and what was the timing of lake growth, decay, and related flooding events? Our preliminary study uses satellite imagery, fieldwork, and historic map work to begin placing constraints on Lake Ahtna spatial parameters. Satellite imagery from ASTER, Landsat, and Google Earth, and field photos show interpreted relict glacial features such as trimlines, nunataks, U-shaped and hanging valleys, drumlins, and ice-truncated spurs that constrain the location, thickness, and relative heights of ice dams and the impounded lake. A regional scale mosaic assembled from the ASTER Global DEM (GDEM) is used to generate transects that reveal lake-related features that correlate with historic mapping results, and reflect paleo lake levels. We also apply the GDEM to generate elevation isolines and derive Lake Ahtna extent, volume, and bathymetry for several likely lake levels that are defined and constrained by the geomorphic features described above. A dynamic and complicated history is indicated; and it is recognized that specific lake margin (terrace and dune) elevations are difficult to correlate across the basin because of the complexity and multiplicity of lake margin and glacial geomorphic features, differing unit susceptibilities to lake terrace formation, and possible differential postglacial and post-lake rebound. Thus, a full reconstruction of the lake will require incorporation of tectonics into sedimentological and geomorphological models. The reconstruction of Lake Ahtna evolution serves as an excellent analog for lake development that may occur with the diminishment of large land-bound ice sheets such as those in Greenland and Antarctica.