The Snake River Plain is a valley in the U.S. state of Idaho. It covers most of the southern part of the state and is transected by the Snake River. The Snake River Plain was formed by magma rising up below the North American Plate. As the plate moved westward, the magma melted rock under the plate. This melting caused the surface to drop and form the Snake River Plain. Underneath the plain is the Snake River Aquifer. The aquifer is fed by the Snake River and its tributaries. Much of the soil in the Snake River Plain is fertile. As a result, today the primary human purpose of the plain is for agriculture.