The Yellowtail Unit in Southcentral Montana is part of the Pick-Sloan Missouri Basin Program. It is a multi-purpose development providing irrigation water, flood control, recreation and power generation. Facilities consist of Yellowtail Dam and Bighorn Lake on the Bighorn River, Yellowtail Powerplant at the toe of the dam, Yellowtail Afterbay Dam--a short distance downstream--and related structures.
The Crow Indian Reservation, spreading over about 3,500 square miles, encompasses the dam site, a portion of the reservoir area. Yellowtail Dam at the mouth of Bighorn Canyon impounds the river for multi-purpose use. Bighorn Lake is about 72 miles long at maximum water surface elevation, 66 miles long at the top of joint-use storage, and extends into the Bighorn basin of Wyoming. The reservoir is confined in the canyon for most of its length.
Madison limestone which due to topographic expression has been divided into three members; lower, middle, and upper. Special treatment: Grouting beneath the dam, grouting in lower, middle, and upper Madison formations of the right and left abutments, supplemental grouting program of the left abutment.