Island Park Dam


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 Dam, Idaho.

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).

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.

in the early 1990's a privately owned powerplant was built at the dam. In the 2000's a one foot inflatable rubber dam was installed on the spillway crest to allow better control of the 1 foot of surcharge.


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.

RegionFacilities in Pacific Northwest | Pacific Northwest Home Page
Dam TypeZoned Earthfill
ReservoirIsland Park
WatercourseHenrys Fork
Original Construction1937-1939
Modified Construction
National ID NumberID00272
Hydrologic Unit Code

Structural Height94.0 ft
Hydraulic Height (Normal Operating Depth at Dam)75.0 ft
Top Parapet (Elevation)
Spillway Crest Elevation6308.2 ft
Crest Elevation6312.0 ft
Crest Length9500.0 ft
Crest Width
Base Width
Volume of Dam Construction Materials
Streambed at Dam Axis6228.0 ft
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)
Top of Active Conservation Pool (Elevation)6303.0 ft
Top of Inactive Conservation Pool (Elevation)6239.0 ft
Top of Dead Storage Pool (Elevation)6230.0 ft

Hydraulics & Hydrology
Total Water Storage at Elevation 135,205 ac-ft
Maximum Water Surface Elevation
Normal Water Surface Elevation6303.0 ft with inflatable bladder, otherwise it is 6302.0 ft
Spillway Type Bathtub spillway with a 1 ft inflatable bladder on top
Auxiliary SpillwayYes
Spillway Capacity at Elevation6,600 cfs at 6306.6 ft
Auxiliary Spillway Capacity at Elevation
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation3,260 cfs at 6303.0 ft
Diversion Capacity at Elevation
Drainage Area482.0 sq mi
Hydrometeorological Report (HMR)
Probable Maximum Flood (PMF) Report

Contact Information
OrganizationUpper Snake Field Office 
Address470 22nd Street
CityHeyburn, ID 83336
Phone (Fax) 208 678-4321 
Phone (Office)208 678-0461
OrganizationFremont-Madison Irrigation District
AddressPO Box 15
CitySt. Anthony, ID 83445-0015
Phone (Office)208 624-3381

Last updated: Nov 29, 2012