Island Park Dam is located 38 miles north of Ashton, Idaho, on Henrys Fork. The dam is a zoned earthfill structure 91 feet high. Water stored at Island Park and Grassy Lake Reservoirs is used in Fremont and Madison Counties in northeastern Idaho, and Teton County in Wyoming. The Upper Snake River Division of the project includes Island Park Dam, Cross Cut Canal and Diversion Dam, and Grassy Lake Dam. Island Park Reservoir has a total storage capacity of 135,500 acre-feet (active 135,200 acre-feet) which includes 1 foot of surcharge that is filled on a recurring basis and is part of the allocated storage space. The Safety of Dams modifications were completed at Island Park Dam in the early 1980s. This consisted of replacing the deteriorated concrete in the spillway and excavation and replacement of liquefiable materials in the right abutment. Geology Island Park Dam rests within the Middle Rocky Mountain- Physiographic Province near the northwest rim of the Island Park Caldera. The dam foundation exhibits the alternating volcanic eruptions of rhyolite and basalt with rhyolite common within the dam's valley section and right abutment. Basalt is found within the left abutment, together with rhyolite. | 
