St. Mary Diversion Dam


The St. Mary's Diversion Works, located on the St. Mary River 0.75 downstream from Lower St. Mary Lake, is a 6-foot-high concrete weir and sluiceway with a length of 198 feet and a total volume of 1,200 cubic yards.

The St. Mary Canal begins at St. Mary Diversion Dam on the west side of St. Mary River and crosses the river 9.5 miles below the diversion through a two-barrel steel-plate siphon 90 inches in diameter and 3,600 feet in length. Eight miles below the St. Mary crossing a second two-barrel steel-plate siphon, 78 inches in diameter and 1,405 feet long, conveys the water across Hall's Coulee. A series of five large concrete drops at the lower end of the 29-mile canal provide a total fall of 214 feet to the point where the water is discharged into North Fork Milk River. Design capacity of the canal is 850 cubic feet per second.

RegionFacilities in Great Plains | Great Plains Home Page
ProjectMilk River
Dam TypeConcrete weir and sluiceway
WatercourseSt. Mary River
Location On the St. Mary River two miles southeast of Babb, Montana.
Original Construction1915
Modified Construction
National ID NumberMT82932
Hydrologic Unit Code

Structural Height6.00 ft
Hydraulic Height (Normal Operating Depth at Dam)6.00 ft
Top Parapet (Elevation)
Spillway Crest Elevation
Crest Elevation4,457.50 ft
Crest Length198.00 ft
Crest Width
Base Width
Volume of Dam Construction Materials 1,200 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)
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 TypeOverflow crest
Auxiliary Spillway
Spillway Capacity at Elevation850.00 cfs at 4,457.50 ft
Auxiliary Spillway Capacity at Elevation
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation
Diversion Capacity at Elevation 850.00 cfs at 4,457.50 ft
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
OrganizationParadise Valley Irrigation District
AddressPO Box 1417
CityChinook, MT 59523
Phone (Office)406 265-8269

Last updated: Apr 02, 2013