Isa Lake located in Yellowstone National Park straddles the continental divide at Craig Pass and was first discovered in 1891 by Lt. Hiram M. Chittenden of the US Army Corps of Engineers while he was exploring routes for a road to connect Old Faithful and the West Thumb Geyser Basin. In times of high water, Isa Lake has two outlets. The waters on the west side of the lake flow east to the Firehole River then westward into the Gulf of Mexico and the waters on the east side flow west to the Lewis River then eastward into the Pacific Ocean. If you trace the Columbia River to its source, you will find one of its tributaries, the Snake River, in Yellowstone National Park. The park is also the source for the Madison River which flows into the Missouri River upstream of Fort Peck Lake and the Yellowstone River which flows into the Missouri River upstream of Lake Sakakawea.