Catastrophic flooding into the Great Lakes from Lake Agassiz

As the margin of the Laurentide Ice Sheet in Ontario retreated northward from the Lake Superior basin, the barrier that had prevented the vast Lake Agassiz watershed from overflowing into the Great Lakes was breached. Although this process must have occurred many times during the Quaternary, and at least twice during retreat of Late Wisconsinan ice, only the series of catastrophic flood bursts associated with retreat of the Marquette glacial advance from the Superior basin after 10,000 B.P. is easily documented.

While ice lay across the divide between the Agassiz and Superior basins, overflow from Lake Agassiz and its 2 × 10⁶ km² watershed discharged south into the Mississippi River system. As ice retreated north along this divide, down the regional slope toward the Hudson Bay basin, a series of cols were uncovered, and water began to spill eastward into the Nipigon and Superior basins. The initial discharge through each outlet increased abruptly when the remaining glacial dam across the channel or channel complex failed. The resulting catastrophic burst of water stripped away glacial sediment and plucked giant slabs of jointed Proterozic diabese, eroding deep channels as it made its way down the steep gradient to the Nipigon basin and then to the Superior basin east of Thunder Bay, Ontario. As the level of Lake Agassiz declined, so did the discharge, reaching an equilibrium flow of about 30,000 m³s^–1. Successive flood bursts occurred as lower cols in the divide were uncovered during northward retreat of the Laurentide ice.

Maximum flows of 200,000 m³s^–1 were estimated using the Manning equation and shear stress calculations based on the size of bedload in these channels. The largest catastrophic bursts discharged 3000–4000 km³ of water from Lake Agassiz into Lake Superior in only a year or two. In turn, this water overflowed east into the Huron basin and then through the North Bay outlet to the Ottawa River Valley and the St. Lawrence Lowland. During these catastrophic floods, the level of Lake Superior may have fluctuated dramatically and flow through the St. Lawrence Valley122 to the Atlantic Ocean increased by up to four times. Many of the numerous beaches and cut strandlines in the Lake Agassiz basin are related to periods of equilibrium flow between these floods.