Agassiz, Lake (ăgˈəsē) [key], glacial lake of the Pleistocene epoch, c.700 mi (1,130 km) long, 250 mi (400 km) wide, formed by the melting of the continental ice sheet beginning some 14,000 years ago; it eventually covered much of present-day NW Minnesota, NE North Dakota, S Manitoba, central E Saskatchewan, and SW Ontario. The lake was named in 1879 in memory of Louis Agassiz for his contributions to the theory of the glacial epoch. Lake Traverse, Big Stone Lake, and the Minnesota River are in the channel of prehistoric River Warren, Lake Agassiz's original outlet to the south. As the ice melted, the water drained E into Lake Superior, and after the ice disappeared, N into Hudson Bay. The lake's disappearance (c.8,400 years ago) left lakes Winnipeg, Manitoba, and Winnipegosis, Red Lake, Lake of the Woods, and other smaller lakes. The bed of the old lake, the Red River valley, has become an important crop-growing region due to its rich soil.