The Red River Lowlands have been repeatedly flooded since retreat of the Laurentide Ice Sheet about 12,000 years ago. Initially that ice sheet formed a dam that prevented drainage of the Canadian Prairies and Northern Great Plains of the U.S. from reaching Hudson Bay. The largest lake in the world, Lake Agassiz, was ponded in front of the glacier for nearly 5000 years, inundating most of southern Manitoba and parts of northwestern Ontario, Saskatchewan, North Dakota, and Minnesota. Sandy beaches formed around the shorelines of the lake when it was at different levels, and clay was deposited in the deeper waters of the basin. Overflow from Lake Agassiz during its life was through several different routes: (1) south into the Mississippi River and Gulf of Mexico, (2) east into the Great Lakes to the St. Lawrence Valley and North Atlantic Ocean, and (3) northwest into the Mackenzie River and the Arctic Ocean. The lake finally drained away into Hudson Bay about 8400 years ago, after which modern drainage patterns were established across central Canada. Each change in routing of Lake Agassiz overflow was accompanied by a catastrophic burst of water; when that burst reached the oceans it changed the way they circulated and, in turn, caused global cooling.
After Lake Agassiz drained away, the modern Red River was established on the floor of the old lake bed. Because of the low gradient of the river and low relief of the lake plain, flooding has repeatedly occurred along the river, inundating large areas during spring runoff; major floods in the 19th century were recorded in 1826, 1852, and 1861. Saturated ground conditions in the fall, early freezing of the ground, high precipitation, and rapid and late snow melt in the spring commonly have led to spring flooding. In recent years, such as 1979, 1996, 1997, 1999, 2006, and 2009, large floods have occurred. Only construction of the 48-km-long Red River Floodway in 1968 around the city of Winnipeg has prevented flood disasters in the city of Winnipeg like the one in 1950; the Floodway prevented an estimated $10 billion in flood damage in 2009.