The Snake River Aquifer is a large reservoir of groundwater underlying the Snake River Plain in the southern part of the U.S. state of Idaho. Most of the water in the aquifer comes from irrigation recharge. Measuring about 400 miles (640 km) from east to west, it is an important water source for agricultural irrigation in the Plain. The Snake River Aquifer is commonly defined as two separate parts, separated by Salmon Falls Creek: the Eastern Snake River Plain Aquifer and Western Snake River Plain Aquifer.