Lake AgassizLargest of the ice-marginal lakes that once covered what are now parts of Manitoba, Ontario, and Saskatchewan in Canada, and North Dakota and Minnesota in the United States. It was present in the Pleistocene Epoch (a geological period that lasted from 1,600,000 to 10,000 years ago) during the last two phases of the Wisconsin Glacial Age, when the Laurentide Ice Sheet blocked the drainage of the northern Great Plains into what is today Hudson Bay. As a result, the waters of the Saskatchewan and other rivers backed up, forming the 700-mi- (1,100-km-) long by 200-mi-wide Lake Agassiz and the smaller lakes of Souris and Saskatchewan, which drained through various outlets (depending upon their water level) either into the Mississippi River (via the Minnesota River) or Lake Superior. With the retreat of the ice sheet, after nearly 1,000 years, a channel to the north (now the Nelson River) drained the 110,000-sq-mi (285,000-sq-km) Lake Agassiz into Hudson Bay, leaving lakes Winnipeg, Winnipegosis, and Manitoba, and Lake of the Woods (qq.v.) as remnants. The fine claylike silt that accumulated on the bottom of Agassiz is responsible for the fertility of the valleys of the Red and Souris rivers. The lake was named in 1879 after the Swiss-born naturalist and geologist Louis Agassiz, who conducted extensive studies on the movement of glaciers.