Helena Valley Dam


Water flowing from Helena Valley Tunnel at mile 2.8 discharges into the 300-cubic-foot-per-second Helena Valley Canal. The Canal is 31.7 miles long, with 10.2 miles unlined and 21.5 miles lined. at mile 11, the canal discharges into a 10,500-acre-foot-capacity regulating reservoir. Helena Valley Dam, which forms the reservoir, is an earthfill structure 91 feet high, with a crest length of 2,650 feet. A 600-foot-long dike extends from the left abutment of the dam.

The reservoir has 5,900 acre-feet of conservation storage for irrigation and municipal water. A outlet built into the dams supplies municipal water to the city of Helena. The 350-cubic-feet-per-second outlet to the Helena Valley Canal is located in the dike. Between the reservoir and mile 17, the canal provides facilities for future supplemental service to 10 irrigation ditches which have been diverting water from Prickly Pear Creek; between miles 17 and 22, facilities are provided for supplemental service to existing ditches diverting water from Prickly Pear and Tenmile Creeks. The canal terminates at Lake Helena, which occupies the lower part of the valley.

The reservoir area upstream from the dam has been earth blanketed to reduce seepage from the reservoir, and pressure relief wells have been installed near the downstream toe of the dam. These measure have not been totally effective and other measures are being considered to relieve the uplift pressures on the downstream toe of the dam.

Foundation consists of an alternating sequence of tuff, tuffaceous sandstone, and conglomerate beds. These sediments are part of the Bozeman beds of Paleocene to the Miocene Age. Numerous faults and fractures cut these strata beneath and adjacent to the dam. One fault with a displacement of about 200 feet trends northward through the right abutment of the dam. Some fault and fracture zones have been healed by clastic material intruded during earthquake. Since initial filling of the reservoir in late 1959, excessive seepage and hydrostatic pressures under the dam have been encountered.

RegionFacilities in Great Plains | Great Plains Home Page
CountyLewis and Clark
ProjectHelena Valley Unit, Pick-Sloan Missouri Basin Program
Dam TypeZoned earthfill
Reservoir Helena Valley
WatercourseTenmile Creek Tributary
LocationApproximately 8 miles northeast of Helena, Montana and 3.5 miles west of Canyon Ferry Dam.
Original Construction1957-1958
Modified Construction 1981: Existing dam embankment modified to stabilize foundation and downstream toe. Relief wells constructed and downstream drainage channel lowered.
National ID NumberMT00743
Hydrologic Unit Code

Structural Height94.00 ft
Hydraulic Height (Normal Operating Depth at Dam)65.60 ft
Top Parapet (Elevation)
Spillway Crest Elevation
Crest Elevation3,828.00 ft
Crest Length2,650.00 ft
Crest Width20.00 ft
Base Width675.00 ft
Volume of Dam Construction Materials 304,000.00 cu yds
Streambed at Dam Axis 3,755.00 ft
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)
Top of Active Conservation Pool (Elevation)3,820.07 ft
Top of Inactive Conservation Pool (Elevation)3,805.00 ft
Top of Dead Storage Pool (Elevation)3,787.75 ft

Hydraulics & Hydrology
Total Water Storage at Elevation10,451.00 acre-ft at 3,820.07 ft
Maximum Water Surface Elevation10,702 acre-ft at 3,820.55 ft
Normal Water Surface Elevation
Spillway Type
Auxiliary Spillway
Spillway Capacity at Elevation
Auxiliary Spillway Capacity at Elevation
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation350.00 cfs at 3,807.00 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
OrganizationHelena Valley Irrigation District
Address3840 N Montana Ave
CityHelena, MT 59601
Phone (Office)406 442-3292

Last updated: Mar 29, 2013