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The Bighorn River is a tributary of the Yellowstone, approximately 461 miles (742 km) long, in the states of Wyoming and Montana in the western United States. The river was named in 1805 by fur trader François Larocque for the bighorn sheep he saw along its banks as he explored the Yellowstone. At the border with Montana, the river turns northeast, and flows past the north end of the Bighorn Mountains, through the Crow Indian Reservation, where the Yellowtail Dam forms the Bighorn Lake reservoir. The reservoir and the surrounding canyon are part of the Bighorn Canyon National Recreation Area.