Sediment Control of Convection in Glacier Dammed Lakes

During August, 2001, measurements of bathymetry, temperature, and conductivity from Berg Lake, a freshwater lake dammed by Steller Glacier, and Vitus Lake, a tidally influenced lake at the terminus of Bering Glacier, Alaska, show intense vertical convection that is controlled by suspended sediment in the former lake and salt in the latter lake. The temperature profiles from Berg Lake show a vertical structure that consists of a 10 m thick surface layer where the temperature drops from near 9 C to approximately 4 C, the temperature of maximum density for fresh water. Below this depth the temperature decreases to 0 C in the deepest portions of the lake, approximately 75 m. Superimposed on this general unstable temperature profile are spatially variable fine structure details that include vertical steps and temperature inversions. While the temperature profiles indicate a highly unstable situation, the sub-glacial discharge has a suspended sediment load sufficient to marginally stabilize the density structure in the lake. This sediment laden water flows out from below the glacier and spreads horizontally throughout Berg Lake. As the suspended sediment settles, vertical thermal convection occurs that yields the observed fine structure in the temperature profiles. In contrast, Vitus Lake is connected to the Gulf of Alaska via the 8 m deep, 7.3 km long Seal River. Measurements in this lake show strong saline stratification in the deeper portions of the lake. Thermal diffusion across the pycnocline may produce frazil ice growth, while melting of the glacier terminus produces convection at the margin of the lake, not the interior, as was observed in Berg Lake.