 With a surface area of (excluding islands; if islands are included), it is the largest bifurcation lake in the world – that is, a lake that drains naturally in two directions.About 10% of the lake's water drains into the Fond du Lac River, which flows out of the lake to the northwest, where it drains into Lake Athabasca, which ultimately drains into the Arctic Ocean via the Mackenzie River system.The rest of the water drains into the Cochrane River, which flows out of the northeastern side of the lake and into Reindeer Lake, which drains via the Churchill River system into Hudson Bay.