History of late glacial runoff along the southwestern margin of the Laurentide Ice Sheet

Abstract

The routing of runoff from the retreating Laurentide Ice Sheet was controlled by a combination of ice-marginal position and topography; isostatic depression also played an important role in the evolution of the ice-marginal landscape. A major redirection of drainage occurred after ice retreated north of the Missouri Escarpment about 12 ka BP. Runoff that had previously flowed south to the Missouri River could then be routed eastward to Lake Agassiz, which in turn drained to the upper Mississippi River valley.Changes in drainage routing in the region west of glacial Lake Agassiz typically were abrupt, as new and lower paths along the retreating ice margin became available. Each new basin was larger than its predecessor and its headwaters extended farther west. Four distinct, sequential phases of ice-marginal drainage occurred between 12 and 10.7 ka BP: the James, Sheyenne, Souris-Pembina and Qu'Appelle-Assiniboine. Starting with the Qu'Appelle-Assiniboine phase about 11.2 ka BP, Lake Agassiz collected runoff from as far west as the Rocky Mountains. By about 10.7 ka BP, ice had retreated north of the lower reaches of the Saskatchewan River, allowing meltwater to by-pass the Qu'Appelle-Assiniboine basin. At about the same time, the eastern outlets of Lake Agassiz opened, initiating drainage to the North Atlantic Ocean from this vast region.Drainage routes evolved by a common sequence of events, beginning with the impoundment of proglacial lakes along the ice margin. Most of these lakes drained catastrophically, resulting in the formation of spillways with distincitve geomorphic features that include broad, scoured and streamlined subupland channels along with subsequently incised narrow, deep inner channels. Characteristic deposits of these outbursts include boulder mantles on subupland channel floors, boulder-gravel bars within the inner channels, and large, primarily subaqueous fans in lake basins that received the flood bursts. These outbursts progressed from basin to basin, causing the drainage of other proglacial lakes, until they reached Lake Agassiz. Spillways associated with the next younger drainage phase were commonly incised across the abandoned lake floors.