Bighorn River, a river in northern Wyoming and southern Montana. It begins at the junction of the Wind and Popo Agie rivers, at Riverton, Wyoming, and flows northward to the Yellowstone River in Montana. The entire course, extending to the headwaters of the Wind River, is about 460 miles (740 km) long; the drainage basin is about 20,000 square miles (50,000 km2). Tributaries include the Greybull, Shoshone, and Little Bighorn. Dams impound several large reservoirs, including Bighorn Lake, site of Bighorn Canyon National Recreation Area.