Restoring Critical Irrigation and Drinking Water to the Montana Hi-Line

The St. Mary Canal project originates in Glacier Park, Montana, continues through a 29-mile canal system including three siphons and five drop structures, and finally discharges into the North Fork of the Milk River. From here, the water continues through Canada before it flows back into the United States. The canal water flows through two tribal nations to deliver irrigation water to 120,000-plus acres and drinking water to 18,000 residents while fostering miles of wildlife habitat and recreational opportunities. In dry years, 90%-100% of the water in the Milk River Basin is diverted from the St. Mary River. During average years, the diverted St. Mary water represents approximately 70% of the Milk River flow from May through September. The U.S. Department of Interior Bureau of Reclamation reported that in the late summer, for 4 out of every 10 years, there is no natural flow in the Milk River and that it would run dry without the diverted St. Mary River water.

On June 17, 2024, the St. Mary Canal Siphon suffered a catastrophic failure. The siphon breach caused the 100-plus-year-old siphons to collapse, washing away the existing concrete pipe supports and severely undercutting the structure. This failure led to washout areas estimated to be 30 to 50 feet deep. 

The Milk River Joint Board of Control, Bureau of Reclamation, the Blackfeet Nation, the State of Montana, HDR, contractors and suppliers quickly convened to determine how to restore critical water supplies. After a thorough assessment, it was decided that a complete replacement of the St. Mary Siphons was necessary to provide reliable water supplies for its served communities and natural resources. 

After initial federal funding was secured to begin site remediation activities, the State of Montana made available approximately $32 million for the Milk River Joint Board of Control to begin work on the siphon replacement with an anticipated completion date of summer 2025.

Our team has been providing design and construction oversight for the replacement of the failed siphons. Work has included the collection of additional field data to support hydraulic analysis; hydraulic modeling to support a design that is hydraulically stable and mitigates the surging and pulsing; design and procurement specifications; and on-site construction administration.