Jackson Lake is the beginning of the Snake River. The lake is nestled at the base of the Teton Mountains within Grand Teton National Park  About 15,300 square miles of the study area is associated with the Snake River Plain with an additional 56,000 square miles within the hydrologic units that extend up the tributary basins.

Water in the Snake River begins its journey flowing south through Jackson Hole, as shown in photo to left..  The major tributaries in this region include the Gros Ventre River, Hoback River, Greys River, and the Salt River.

More than twenty tributary streams feed into the Snake River in the eastern portion of the Snake River basin. The larger tributaries in this area include: Henrys Fork, Teton River, Falls River, Big Wood River, Blackfoot River, Big Lost River, and Portneuf River. The Snake River drops in elevation from more than 5,015 feet on the eastern side of the plain, to about 2,100 feet on the western edge near Weiser, Idaho.

The most downstream reach for this study, the Middle Snake-Boise sub basin, extends from King Hill to Brownlee Dam and includes the western Snake River Plain. The western Snake River Plain is about 50 miles wide and 145 miles long (Newton, 1991). The major tributaries in this region include: Payette River, Boise River, Weiser River, Owhyee River, Bruneau River, Malheur River, Powder River, and Burnt River.