Willow Creek Dam


Willow Creek Dam is an earthfill structure on Willow Creek about 15 miles southeast of Gibson Dam. In addition to storing water from Willow Creek, the reservoir is fed from the Sun River through the Willow Creek Feeder Canal. The structure is 93 feet high, has a crest length of 650 feet, and contain 275,000 cubic yards of material. An open spillway channel 700 feet wide at the ground surface has a capacity of 10,000 cubic feet per second. The outlet works tunnel run through the right abutment. The reservoir has a capacity of 32,400 acre-feet of water.

Geology at the dam site consists of Cretaceous age bedrock of the Two Medicine Formation overlain by Pleistocene glacial moraine deposits and recent alluvium and colluvium along the channel of Willow Creek. Bedrock consists of nearly horizontal bedded, decomposed to soft shales and siltstones, and fine-grained sandstones. The shales and siltstones are usually poor laminated and tend to air slake rapidly. The sandstones are moderately hard to hard, and high angle jointing is prominent and leads top formation of blocky, tabular fragments. Glacial moraine deposits in the area are quite coarse and generally consist of sand and gravel with minor amounts of cobbles, boulders and silt and clay fines. Alluvium along the creek channel is a highly variable mixture of material ranging in size from clay fines to boulders. A thin layer of material at the base of the dam, identified as alluvium, is composed of mostly cobbles and gravel with almost no fines. Glacial deposits form the foundation of the dikes and spillway.

RegionFacilities in Great Plains | Great Plains Home Page
CountyLewis and Clark
ProjectSun River
Dam Type Modified homogeneous earthfill
ReservoirWillow Creek
WatercourseWillow Creek
LocationOn Willow Creek, tributary to the Sun River, in Lewis and Clark County about 5 miles northwest of Augusta, Montana. Four small earth dikes close low areas north of the dam.
Original Construction1907-1911
Modified Construction Modifications made in 1917; 1941; 1996-1998; and 1999-2000. Modified in 1999-2000 to include a filtered berm and buttress.
National ID NumberMT00577
Hydrologic Unit Code

Structural Height93.00 ft
Hydraulic Height (Normal Operating Depth at Dam)69.00 ft
Top Parapet (Elevation)
Spillway Crest Elevation4,144.00 ft
Crest Elevation4,154.00 ft
Crest Length650.00 ft
Crest Width 30.00 ft
Base Width 456.00 ft
Volume of Dam Construction Materials 366,000.00 cu yds
Streambed at Dam Axis
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)
Top of Active Conservation Pool (Elevation)4,142.00 ft
Top of Inactive Conservation Pool (Elevation)4,085.28 ft
Top of Dead Storage Pool (Elevation)4,085.28 ft

Hydraulics & Hydrology
Total Water Storage at Elevation32,300.00 acre-ft at 4,142.00 ft
Maximum Water Surface Elevation 4,144.00 ft
Normal Water Surface Elevation
Spillway TypeUncontrolled, open channel emergency spillway in dike #5 about 3,600 ft north of dam with concrete cutoff wall and rock paving at crest.
Auxiliary Spillway
Spillway Capacity at Elevation10,000.00 cfs at 4,147.00 ft
Auxiliary Spillway Capacity at Elevation
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation500.00 cfs at 4,142.00 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
TitleArea Manager
OrganizationMontana Area Office
AddressP.O. Box 30137
CityBillings, MT 59107-0137
Phone (Fax)406 247-7338
Phone (Office)406 247-7300

Last updated: Apr 02, 2013