Gibson Dam


Gibson Dam, the principal structure of Sun River Project, is on the Sun River, 70 miles west of Great Falls, Montana. It is a concrete arch dam 199 feet high and contains 167,500 cubic yards of concrete. A drop inlet spillway located just upstream from the north end of the dam has a capacity of 30,000 cubic feet per second. The entrance to the shaft and 29.5-foot diameter tunnel is controlled by six 34- by 12-foot radial gates. Outlet structures for the dam are two 72-inch diameter semisteel-lined conduits through the base of the dam. The maximum capacity of the outlets is 3,050 feet per second. Gibson Reservoir has a total capacity of 99,100 acre-feet.

Crystalline limestone in regular beds which strike crosswise of river. Special treatment: cement-grout curtain beneath upstream cutoff trench; spring and left abutment grouted.

RegionFacilities in Great Plains | Great Plains Home Page
CountyTeton, Lewis and Clark
ProjectSun River
Dam TypeConcrete arch
WatercourseNorth Fork of Sun River
LocationOn the North Fork of Sun River about 24 miles northwest of Augusta, Montana.
Original Construction1926-1929
Modified ConstructionSpillway modified in 1938; outlet works gates replaced in 1971; overtopping modification in 1982
National ID NumberMT00571
Hydrologic Unit Code

Structural Height199.00 ft
Hydraulic Height (Normal Operating Depth at Dam)195.00 ft
Top Parapet (Elevation)4,729.00 ft
Spillway Crest Elevation4,712.00 ft
Crest Elevation4,725.50 ft
Crest Length960.00 ft
Crest Width15.00 ft
Base Width117.00 ft
Volume of Dam Construction Materials167,500.00 cu yd
Streambed at Dam Axis
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)
Top of Active Conservation Pool (Elevation)4,724.00 ft
Top of Inactive Conservation Pool (Elevation)
Top of Dead Storage Pool (Elevation)4,557.50 ft

Hydraulics & Hydrology
Total Water Storage at Elevation96,477.00 acre-ft at 4,724.00 ft
Maximum Water Surface Elevation 4,741.00 ft
Normal Water Surface Elevation
Spillway TypeDrop-inlet, discharging into shaft and tunnel in left abutment, controlled by 6 34- by 12 - ft radio gates.
Auxiliary Spillway
Spillway Capacity at Elevation50,000.00 cfs at 4,729.00 ft
Auxiliary Spillway Capacity at Elevation
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation 3,050.00 cfs at 4,724.00 ft
Diversion Capacity at Elevation
Drainage Area
Hydrometeorological Report (HMR)
Probable Maximum Flood (PMF) Report

Contact Information
TitleArea Manager
OrganizationMontana Area Office
AddressP.O. Box 30137
CityBillings, MT 59107-0137
Phone (Fax)406 247-7338
Phone (Office)406 247-7300
OrganizationGreenfields Irrigation District
AddressBox 157
CityFairfield, MT 59436
Phone (Office)406 467-2533

Last updated: Mar 29, 2013