Near the end of the Pleistocene Epoch, as the last of the Continental Ice Sheets was retreating into Canada, a massive lake of meltwater formed along the southern margin of a glacial lobe, covering much of southern Manitoba and a swath of America's Northern Plains. Known as Glacial Lake Agassiz, its surface area fluctuated over thousands of years; during its maximum extent (some 11-12,000 years ago), the lake's southern arm extended down the border of present-day North Dakota and Minnesota and Lake Agassiz drained southeastward through the valley of the Minnesota River.
As the ice continued to recede, the meltwater shifted northward as well, finding outlets to the east and gradually abandoning its southern arm. About 9,000 years ago, the Red River began to form in that broad lake bed, meandering northward to the ever-changing contour of Lake Agassiz. Eventually, when the glacial ice had retreated into northern Canada and the Northern Plains began to rebound from its weight, the Red River emptied into Lake Winnipeg, the largest remnant of Lake Agassiz; Lake Winnipeg now drains into Hudson Bay via the Nelson River.
Today, the Red River snakes northward across the flat bed of Lake Agassiz, covering 545 miles on its journey from Wahpeton, North Dakota, and Breckenridge, Minnesota, to Lake Winnipeg. Bordered by rich agricultural land and flowing past several large cities (Fargo, Grand Forks and Winnipeg), the river is highly prone to flooding, especially in late winter or early spring when the frozen ground and dormant vegetation cannot absorb excess moisture. Snowmelt and heavy spring rains overwhelm the capacity of the shallow river and floodwaters spread across the ancient lake bed, inundating farms, towns and urban areas.
