BABB, Montana — On June 17, 2024, both St. Mary River siphon barrels experienced structural failures, located approximately 8 miles north of Babb, Montana. The breach caused severe erosion, resulting in washout areas at the breach site, impacting the hill and concrete footings holding the pipes. Diversions from the St. Mary River to the Milk River were stopped immediately in response, and the timing of any repairs and future diversions are unclear at this time.
The St. Mary Canal is a cross-basin diversion that supplies the main water source for the Milk River Project irrigators and municipalities, comprising over 60-percent of the water supply in an average year, more than 80-percent in a dry year. Currently, storage in Fresno and Nelson Reservoirs are planned to be used to provide continued irrigation deliveries in the near term until storage is depleted to approximate elevation 2555, which equates to approximately 27,000 acre-feet remaining in the reservoir. Once storage reaches this level operations will transition to conserving storage for meeting municipal water needs and Tribal storage deliveries. This will impact irrigation deliveries to approximately 110,000 acres.
Reclamation has water supply contracts with several communities along the Milk River for municipal and industrial water including Havre, North Havre, Chinook, and Harlem. Water supply contracts total approximately 4,600 acre-feet. Reclamation will be operating Fresno Reservoir to ensure water will continue to be available to these communities along the Milk River throughout the duration of this event, including long term replacement of the siphons.
Lake Sherburne Dam and Reservoir are located upstream of the canal failure and therefore are unable to provide water supply to the Project from the St. Mary River until repairs are completed.
Ryan Newman, Reclamation's Montana Area Office Manager, stated, "Impacts from this failure will be numerous and wide ranging, and we will be working closely with our partners to do everything in our power to minimize impacts."
The St. Mary siphon is a critical component of the Milk River Project, designed to transport water from the St. Mary River across the valley. It consists of two 90-inch riveted steel barrels traverse the valley from the inlet, transition to an 84-inch diameter at the river crossing, and then back to 90 inches in diameter as they ascend the valley slope to the outlet. The siphon has a maximum head of 165 feet and was constructed in two phases, with the downstream barrel completed between 1912-15 and the upstream barrel after 1925.
The siphon has undergone extensive repairs due to seepage, corrosion, and buckling. A cathodic protection system was installed in the 1950s to address these problems. However, unstable valley sidewalls have caused further movement of the steel barrels and concrete supports, leading to additional damage. Reclamation and the MRJBOC initiated designs for replacement siphons in early 2023, which will facilitate replacement in a more timely manner.
