Paleohydrology of the Western Outlets of Glacial Lake Duluth

Abstract

Glacial Lake Duluth occupied the western end of the Lake Superior Basin, dammed between the retreating Superior lobe and a series of moraines. Lake Duluth is identified by a series of discontinuous strandlines observed throughout the western portion of the lake basin. Two prominent outlets have long been recognized, the Portage outlet in Minnesota, which drained southward along the Kettle channel, and the Brule outlet in Wisconsin, which drained along the St. Croix channel. However, the relative role of each outlet in the drainage of the lake has never been adequately explained. The Brule and Portage outlets formed early during ice retreat and they drained small ice marginal lakes. Further ice retreat allowed the small lakes to coalesce forming Lake Duluth. Because of isostatic tilting, the Lake Duluth strandlines rise about 0.5 meters per kilometer eastward between the Portage and Brule outlets from 323 m to 335 m. After adjustment for rebound, the Brule outlet is estimated to be about 10 m below the elevation of the Portage outlet. The paleodischarge of the outlets and their associated channels was estimated using the U.S. Army Corps of Engineers water surface modeling package, HEC-2. Inputs to the model included topographic cross-section of the outlets and channels constructed at 1500 meter spacing from the lake outlets to a distance of 60 km downstream. Analysis of glaciofluvial sediments were used to estimate a range of Manning's roughness coefficients. The model was run with various discharges to construct stage/discharge relations. Maximum lake discharges were detennined by minimum channel cross sections. To check the validity of the discharge estimates, an atmospheric energy-balance approach utilized to estimate potential maximum meltwater availability. Peak channel discharges estimated using HEC-2 and peak summer meltwater production form the energy-balance approach are in remarkably close agreement, ranging from 30,000- 45,0000 m3/s. The Brule outlet served as the primary drainage channel for Lake Duluth. Stage/discharge relations indicate that the Portage outlet could only have been active during the peak of seasonal meltwater production or because of extraordinary inputs of water.