Pishkun Dikes


Pishkun Reservoir and enclosing dikes are founded on glacial deposits (till) of an outwash plain lying east of the Sawtooth Mountains. The thickness and character of glacial till vary widely over short distances. Till was deposited by advancing ice and runoff fed by melting alpine glacier. The thickness range is suggested by the hummocky topography characterized by rounded hills and undrained or poorly drained depressions. The landscape just west of Pishkun Reservoir is dotted with small lakes and is generally poor drainage. The reservoir and dikes are at the eastern margin of the glacial outwash plain. Locally, erosion has cut through glacial deposits exposing shale and sandy shale of the Two Medicine formation. The thickness range of glacial deposits under and around the reservoir is not well known but may range up to 100 feet.

Pishkun Reservoir is an offstream storage reservoir, about 15 miles northeast of Gibson Dam, and has a capaicity of 46,700 acre-feet. The reservoir is part of the Sun River Project and is formed by eight earthfill dikes with heights ranging from 12 to 50 feet with an overall length of 9,050 feet. The outlet for the reservoir is a 12-foot-diameter, approximately semi-circular, concrete conduit through Dike No. 4. The outlet structure has a maximum capacity of 1,600 cubic feet per second. There is no spillway for the reservoir.

RegionFacilities in Great Plains | Great Plains Home Page
ProjectSun River
Dam TypeZoned earthfill
WatercourseOffstream Storage
LocationEight dikes, located offstream about 16 miles southwest of Choteau, Montana
Original Construction1930-1931
Modified ConstructionEnlarged in 1941
National ID NumberMT00575
Hydrologic Unit Code

Structural Height12.00-50.00 ft
Hydraulic Height (Normal Operating Depth at Dam)38.00 ft
Top Parapet (Elevation)
Spillway Crest Elevation
Crest Elevation4,380.00 ft
Crest Length9,050.00 ft (total all 9 dikes)
Crest Width20.00 ft
Base Width
Volume of Dam Construction Materials 599,000 cu yds (total all 9 dikes)
Streambed at Dam Axis
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)
Top of Active Conservation Pool (Elevation)
Top of Inactive Conservation Pool (Elevation)
Top of Dead Storage Pool (Elevation)

Hydraulics & Hydrology
Total Water Storage at Elevation
Maximum Water Surface Elevation
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 Elevation1,600.00 cfs
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: Apr 02, 2013