St. Mary Diversion Dam – Case Study of a 100 year Old Diversion

Abstract

After 100 years of service, the St. Mary Diversion Dam is being replaced with a new design that will provide more effective fish passage and protection. The diversion is located in northern Montana, adjacent to the east side of Glacier National Park. It diverts water from the St. Mary River 1.21 km (0.75 mile) downstream of Lower St. Mary Lake. The existing 60.35-meter-long and 1.83-meter-high (198-foot-long and 6-foot-high) concrete weir diverts up to 24 cubic meters per second (850 cubic feet per second) design flow from the St. Mary River into the North Fork of the Milk River through a 46.67-km (29-mile) long canal, siphon, and drop system. The existing facility, which is maintained and operated by the U.S. Bureau of Reclamation (Reclamation), was completed in 1915 and is in need of replacement. The Endangered Species Act (ESA) listed species that have been adversely affected by limited passage and entrainment into the canal. The St. Mary Diversion and Milk River Irrigation project have been identified as the primary threat to Bull trout habitat in the St. Mary River drainage. Recovery efforts to restore Bull trout habitat in the St. Mary River drainage requires that Reclamation provide effective Bull trout passage and protection at the St. Mary Diversion. This paper presents a short history of the St. Mary Diversion and Milk River Irrigation project, which includes collaborations with all interested parties as well as modeling efforts that were used to enhance the design of the replacement structure to provide the best situation for Bull trout recovery without limiting diversion capability.