Water naturally flows downhill towards the sea. At a continental divide, water on each side of the divide will flow towards a different sea, sometimes heading to destinations hundreds or even thousands of kilometres apart. Occasionally a divide will pass through a standing body of water such as a pond or a lake, and the largest of these is Wollaston Lake in Saskatchewan, Canada, with a surface area of 2,286 km².
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Wollaston Lake drains both into the Arctic Ocean and Hudson Bay. In the two images below I’ve circled its approximate location.
Drainage to the Arctic Ocean starts with the Fond du Lac River, which springs from Wollaston Lake’s northwestern side. From there, water flows into Lake Athabasca, and then into the Mackenzie River system, and finally into the Arctic Ocean.
However if water flows out of the northeastern side of the lake it will flow into the Cochrane River and then Reindeer Lake, and finally into the Churchill River system which completes its journey at Hudson Bay. This is quite a bit shorter than the other route.Wollaston Lake presents a rather unusual situation where a continental divide remains rather fluid, literally, and shifts based on currents rather than the contours of the land.
Source of Mackenzie River Basin map: http://commons.wikimedia.org/wiki/Image:Mackenzie_River_drainage_basin.PNG. Llicensed under the Creative Commons Attribution ShareAlike 2.5 License.Source of Churchill River Basin map: http://en.wikipedia.org/wiki/Image:Churchill-bassin.png. Released into the public domain.