river, southeastern Idaho, U.S., that rises in Henrys Lake in Caribou-Targhee National Forest, near the Montana line, and flows south and southwestward, past St. Anthony to join the Snake River near Rexburg after a course of about 117 miles (188 km). Island Park Dam (1938) and Island Park Reservoir, which the dam impounds north of Upper and Lower Mesa Falls, are used for irrigation. The river is named for the pioneer trapper and trader Andrew Henry, who in 1810 constructed a winter post called Fort Henry near present-day St. Anthony.