CONTRASTING CHARACTERISTICS OF THREE ICE-CONTACT LAKES AT SHERIDAN, BERING, AND MENDENHALL GLACIERS, ALASKA

Moraines constrain Sheridan and Mendenhall Lakes, whereas the origin of Tsiu Lake basin, Bering Glacier, remains unresolved. A repeat bathymetric survey of Sheridan Lake confirmed 1999 data and provided new information from previously inaccessible segments of the basin. A persistent anomalous deepening of unknown origin within a wedge-shaped, ice-front declivity suggests low rates of sedimentation, as deduced from very low turbidity (<0.01 g/L). Water temperature uniformly ranges between (0.4-0.6°C) throughout the lake.

Tsiu Lake, monitored annually since 1990, shows pre-surge, surge, and post-surge changes in bathymetry, temperature, and turbidity. Venting of supercooled water common to other segments of the ice front is lacking and lake turbidity has diminished to 1.5/ g/L. Sediment influx from delta aggradation appears responsible for bathymetrically-determined, increased annual accumulation (3 m/yr to 9 m/yr) as delta growth continues faster than ice front retreat, thus threatening to fill the lake.

Uniformly-low turbidity in Mendenhall Lake (0.1 g/L) may be attributed to a granitic provenance, although bathymetry suggests that sedimentation keeps pace with retreat. However, exceptionally low turbidity in Sheridan Lake must have alternate significance, because it occupies a graywacke terrain from which high sediment yield would be anticipated. Therefore, a lack of subglacial water movement is speculated.

These data indicate that although the general setting and sedimentary environment for each lake is similar, each location has its own set of conditions that reflect the influence of local controls.