Lake Manitoba, 1,817 sq mi (4,706 sq km), SW Man., Canada; one of the largest lakes of North America. A remnant of glacial Lake Agassiz, it is fed by Lake Winnipegosis and drains into Lake Winnipeg. Its shores are marshy. The lake has commercial fisheries and resort areas. The Portage Diversion was built (1970) to redirect floodwaters from the Assiniboine River to the lake.