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Concrete drop structure fails on the St. Mary Canal

Concrete drop structure fails on the St. Mary Canal
Posted at 8:45 AM, May 20, 2020
and last updated 2020-05-20 10:45:46-04

The U.S. Bureau of Reclamation said in a news release that on Sunday, May 17, a concrete drop structure failed on the Bureau of Reclamation’s Milk River Project at St. Mary Canal, northwest of Browning on the Blackfeet Indian Reservation.

The agency says that when the damage was reported, the canal was flowing approximately 200 cubic-feet-per-second (CFS) which is about 1/3 of the canal’s total capacity of approximately 600 CFS. No injuries were reported, and canal flows have been shut off.

This concrete drop structure is the last of five drop structures that use gravity and siphons to convey water through the 29-mile long St. Mary Canal to the North Fork of the Milk River. Water is diverted into the canal from the St. Mary River, near Glacier National Park.

The Bureau of Reclamation, along with stakeholders of the Milk River Project, are assessing the situation to determine the impacts to the water users and options for restoring canal operation. Currently, Fresno Dam and Nelson Reservoir are both at full storage levels and will be used to provide continued irrigation deliveries. Once a plan and schedule for the repair of the canal are developed, water deliveries will be assessed and communicated.

The majority of construction of the Milk River Project was completed between 1906 and 1940. The canal was constructed between 1907 and 1915 and is the primary water source for eight irrigation districts tribes, contract pumpers, and several municipalities downstream of Havre serving approximately 110,000 acres of land.

The agency website provides the following information:

  • The St. Mary's Diversion Dam, 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 the 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.

We will update you when we get more information.