Winnipegosis, Lake, a lake in western Manitoba, Canada. It is one of several interconnected large lakes, including Lakes Manitoba and Winnipeg. All are remnants of an ancient glacial lake named Lake Agassiz. The name Winnipegosis means little Winnipeg. With an area of 2,075 square miles (5,374 km2), the lake is about 125 miles (200 km) long and 25 miles (40 km) wide. Lake Winnipegosis drains into Lake Manitoba through Waterhen Lake and the Waterhen River. Large quantities of pike and whitefish are caught in the lake each year. Lake Winnipegosis was discovered in the 1740's by Sieur de la VĂ©rendrye.