Yellowstone National Park covers a massive volcanic plateau in northwestern Wyoming; slivers of the Park also extend into southern Montana and eastern Idaho. This plateau, which has an average elevation of 8000 feet, is sculpted by a vast network of streams which, ultimately, feed two of North America's largest watersheds.
The Yellowstone River, a major tributary of the Missouri, rises in the Absaroka Mountains near the southeast corner of the Park. Flowing NNW, it enters Yellowstone Lake and then drains that large body of water, dropping through its Upper and Lower falls and carving spectacular canyons before reaching southwestern Montana. The Lamar River drains the northeastern corner of Yellowstone, rising in the Absarokas and flowing westward to merge with the Yellowstone River near the northern boundary of the Park. The Gallatin and Madison Rivers rise in their respective ranges in the northwestern corner of Yellowstone, flowing northwest into Montana where they join the Jefferson River to form the Missouri. Finally, the southwestern region of the Park is drained by the Upper Snake River and its tributaries; the latter include the Lewis River, which connects Shoshone and Lewis Lakes before continuing southward to the Snake and the Heart River, which leaves Heart Lake and flows southwestward to the Upper Snake River. The Continental Divide winds through Yellowstone, separating the rivers that flow into the Missouri River Watershed (the Yellowstone, Lamar, Gallatin and Madison Rivers) from the Snake River tributaries which are part of the Columbia River Watershed of the Pacific Northwest.
While these rivers are vital to the ecology of Yellowstone National Park, they also serve as important migration corridors for resident herbivores and the predators that follow those herds. As winter sets in, herds of bison, elk, mule deer and pronghorn leave the high plateau, traveling through the river valleys on their way to lower, more sheltered grasslands; come spring, their trek through these natural corridors is reversed as the herds take advantage of the short but verdant summer atop the Yellowstone Plateau.
