THE EVOLUTION OF GLACIAL LAKES IN THE WINOOSKI RIVER VALLEY, VERMONT

The Winooski River and its tributaries drain a large portion of north-central Vermont. Water flowing generally WNW out of the Wiinooski River drainage basin was dammed by the retreating Laurentide ice sheet for over 300 years. The existence of proglacial lakes in the Winooski River valley was recognized by some of the first geologists working in the area, e.g. Merwin, (1908). Subsequent work, including the mapping undertaken to produce Vermont’s State Surficial Geologic map in 1970, led to multiple names given to the same glacial lakes and a confusing and scientifically unsound history. Larsen’s work (1972, 1987) in the Winooski River valley and its tributaries presented the first coherent history of glacial lakes that was consistent with mapped lacustrine materials and known outlets.

Our understanding of these lakes has been considerably refined by (1) detailed mapping over the last 30 years, (2) the resurrection and refinement of the North American Varve Chronology, and (3) the recent availability of LiDAR DEM’s. Glacial Lake Winooski, the largest and most long-lived of these lakes, drained through Williamstown Gulf into an arm of Glacial Lake Hitchcock in the White River valley. Several nearly complete varve records indicate that this lake formed ~14,100 years ago and rapidly grew as the ice sheet retreated (>200 m/a) across the area. When a lower outlet was uncovered ~13,800 years ago, lake level quickly fell ~60 m creating the relatively short-lived Glacial Lake Mansfield. Continued westward retreat removed the ice dam from the Winooski River valley allowing water levels to drop another ~70 m to the elevation of the Coveville Stage of Glacial Lake Vermont.

Deltas and beaches have been much easier to locate and map using shaded-relief maps based on LiDAR DEM’s and accurate delta elevations are easy to acquire. Very few deltas expose topset/foreset contacts, however delta terraces (underlain by topset beds) gently slope into the lake. The lowest elevations on Glacial Lake Winooski deltas were used to estimate the different elevations of the lake surface across the area. The orientation of isostatic uplift has been calculated to be 1.15 m/km to N17W (343).