File:Isa Lake (Craig Pass, Yellowstone, Wyoming, USA) 11 (43549145544).jpg

Isa Lake is a small, elongated lake that sits on the Continental Divide at Craig Pass in Yellowstone, Wyoming, USA. It is the only body of water in the world that occurs at a continental divide. When in overflow, part of Isa Lake drains into rivers that, ultimately, lead to the Gulf of Mexico, while the other part of the lake drains into Pacific Ocean.

From park signage:

The Continental Divide [is an] imaginary line [that] winds it sway through the mountains from Alaska to Mexico, separating waters that flow into the Pacific Ocean from those that flow into the Atlantic Ocean.

Every continent except Antarctica has a continental divide, and a few have more than one. While water from these divides normally flows to an ocean, it sometimes becomes trapped in a basin where it can only escape through evaporation or seepage. These areas are known as endorheic basins.

Big Deal for a Little Lake

Other than the brief summer months spent carpeted by beautiful yellow pond lilies, this sliver of water seems rather insignificant. Looks can be deceiving!

As spring runoff empties into Isa Lake, the water swells and overflows its tiny borders. When this happens, the lake does something extraordinary. Not only does it empty into two oceans, but it does so backwards. The east drainage flows into the Pacific, while the west drainage makes the long trek to the Gulf of Mexico.

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