Bighorn, river, 461 mi (741 km) long, formed in W central Wyo. by the confluence of the Wind and Pop Agie rivers and flowing north to join the Yellowstone River in S Mont. The Bighorn basin, part of the Missouri River basin project, has several dams that provide for flood control, irrigation, hydroelectricity, and recreation. Boysen and Yellowtail are the principal dams; the lake behind Yellowtail dam is the nucleus of Bighorn Canyon National Recreation Area (see National Parks and Monuments, table). In 1807 a U.S. trading post was established at the mouth of the Bighorn. The battle between the forces of Col. George Custer and the Sioux took place (1876) near the junction of the Bighorn and the Little Bighorn rivers.