Agassiz, Lake, a prehistoric glacial lake of North America. It existed near the end of the Pleistocene Epoch, or Ice Age. Lake Agassiz centered around what is now Lake Winnipeg and covered parts of present-day Manitoba, Ontario, Saskatchewan, North Dakota, and Minnesota. At its greatest extent, the lake was about 650 miles (1,050 km) long, north to south, and up to 400 miles (640 km) wide. Total area was roughly 100,000 square miles (260,000 km2), larger than that of the Great Lakes combined.

Lake Agassiz was formed when numerous northward-flowing rivers, including the Red River of the North, were blocked by the southern end of a continental ice sheet. For a thousand or more years the temporary ice dam impounded their water as well as water that was melted from the glacier. At that time the lake discharged southward into the Mississippi River by way of the Minnesota River Valley. Eventually the ice sheet retreated far enough north to permit Lake Agassiz to drain into Hudson Bay. Remnants of this once giant body of water include Lakes Winnipeg, Winnipegosis, and Manitoba, and Lake of the Woods. Its former bed is a belt of rich farmlands. It was named in 1879 for Louis Agassiz.