Isa lake: The Two-Ocean Lake

 Isa Lake Viewpoint, located about 8 miles east of the Old Faithful Area along Grand Loop Road in Yellowstone National Park, isn't a very exciting place. On your way to Old Faithful Geyser, you can get off your vehicle and stand at the edge of the water filled with sloughing water and fallen logs. There is a Continental Divide elevation sign along the lake, and an interpretive sign describing the Continental Divide and the significance of Lake Isa.

The 'continental divide' is a geographic division on a continent, often in the form of a mountainous ridge, such that rivers and lakes flow from one side into an ocean or sea, and from the other into another. Isa Lake straddles the continental divide of North America, standing at more than eight thousand feet in the upper watersheds for the two most extensive drainage systems of the Americas—the Snake and Columbia rivers, and the Missouri and Mississippi.

The lake has two outlets. The outlet on the east side of the lake drains into the Pacific Ocean via the Snake and Columbia Rivers and then the Lewis River. The outlet on the west side of the lake feeds the Firehole River on its way to the Missouri and Mississippi Rivers and into the Gulf of Mexico, in other words, the Atlantic Ocean.

Thus Isa Lake is the only natural lake in the world to flow into two different oceans. For this reason it is also known as Two-Ocean Lake. The outflow of the lake is also reverse, as the Atlantic Ocean lies to the east of the lake and the Pacific Ocean to the west.

Isa Lake has no inlet, and is fed entirely by ice-melt. During most of the year, except for spring runoff, the lake's water flow mostly percolates.