GIS-BASED VOLUME AND DISCHARGE CALCULATIONS FOR A CATASTROPHIC FLOOD EVENT FROM GLACIAL LAKE VERMONT INTO THE HUDSON RIVER VALLEY

Strandline evidence from both the New York and Vermont sides of the Champlain Valley indicate at least two separate glacial lacustrine phases: a high-level early ("Coveville") phase, and a later lower-level ("Fort Ann") phase.  Both phases drained southward into the Hudson River Valley across the modern Champlain Valley/Hudson Valley drainage divide.  Although the exact locations of the outlets have not been sufficiently identified, the Fort Ann level outlet was probably at Fort Ann, NY, and the Coveville level outlet may have been there as well.  Exposures of lacustrine sediment in the basin suggest that the drop from Coveville to Fort Ann level (roughly 30 m) was abrupt.  This implies that a large pulse of freshwater must have discharged into the North Atlantic through the Hudson River Valley.  We have undertaken a preliminary calculation of this water volume change and discharge.  The areal extents of the two lake phases were defined using a series of 7.5 minute quadrangle digital elevation models covering the basin.  The average strandline elevation for each lake phase within a quadrangle was estimated using strandline elevations and isobase curves from field observations and published data, and assigned as the phase level for the entire quadrangle.

The ice margin at end of the Coveville phase was at Plattsburgh, NY/Miltonboro, VT.  We estimate that the areal extent of the maximum Coveville phase (from this margin to the outlet at Fort Ann, NY) was about 4,100 km², while the areal extent of the initial Fort Ann phase was about 3,100 km ².  A water volume change of about 108 km³ is estimated by multiplying the average of these extents by the 30 m lake level drop.  If the duration of the volume change event was from one to four months, then discharge from the lake would have been from about 4.5 x 10⁴ m³ /sec (0.045 Sv) to 1.1 x 10⁴ m³/sec (0.011 Sv).