Yellowtail Dam is a dam across the Bighorn River in southwestern Montana in the United States. The mid-1960s era concrete arch dam serves to regulate the flow of the Bighorn for irrigation purposes and to generate hydroelectric power. The dam and its reservoir, Bighorn Lake, are owned by the U.S. Bureau of Reclamation.
The project was the result of negotiations between the federal government and the Crow Nation, the tribe of Native Americans that lived on the surrounding Crow Indian Reservation, and was originally envisioned as a shared facility that would provide profits for both sides. Eventually, the land was sold to Reclamation, although much of the reservoir, which extends 72 miles (116 km) upstream into Wyoming, lies in the reservation. The dam was authorized in 1944 and groundbreaking was in 1961; it was completed in 1967 after six years of construction.
Today aside from its original purposes the dam serves for recreation both above and below the structure. Regulation of the Bighorn provided by the Yellowtail Dam has transformed the lower river into one of Montana's premier trout streams. However, there has been significant controversy surrounding the allocation of water in the reservoir between Montana and Wyoming, and the ecological damage wrought on 184 miles (296 km) of river both above and below the dam.
In the early 20th century, the population of the Yellowstone River valley of southern Montana, of which the Bighorn River is the largest tributary, was growing rapidly and so was the acreage of irrigated land – however, the system was vulnerable to floods and droughts. In 1905, the federal government conducted the first feasibility studies for a dam on a stretch of the Bighorn within the Crow Indian Reservation, some 45 miles (72 km) southeast of Billings, Montana. The leaders of the Crow Nation, which owned the land, agreed to building a dam there because electricity generated there would provide income for the tribe.