Milk River water diversion issues could affect fisheries in Fresno, Nelson and the Milk River
By Moosetrack Megan

Posted: July 15, 2020

The Milk River is the lifeblood of north central Montana. Not only does it provide essential irrigation, it provides important fisheries and wildlife habitat. However, recent failure of the Bureau of Reclamation (BOR) water delivery structures on the St. Mary Canal near the North Fork of the Milk River may mean reliable water flow is in jeopardy this summer. Montana Fish, Wildlife & Parks is working with BOR to monitor this situation and its potential impacts to Fresno and Nelson reservoirs and the Milk River drainage.

On Sunday, May 17, the concrete drop structure number 5 failed on the St. Mary Canal northwest of Cutbank. This concrete drop structure is the last of five drop structures of the St. Mary Canal that use gravity and siphons to carry water 29 miles from the St. Mary River to the North Fork of the Milk River. Water is diverted into the canal from the St. Mary River, near Glacier National Park. This irrigation project, built in the early 1900s, is the main reason the Milk River sustains year-round flows.

Moving forward, the Milk River Joint Board of Control, with the assistance from BOR and the Montana Department of Natural Resources and Conservation, plans to complete a full replacement of Drop 5 and Drop 2, which was considered a high-risk structure prior to the Drop 5 failure. The Joint Board and HDR Engineering will manage the construction project and has selected Sletten Construction Companies to perform the construction. Construction schedules are still being developed but the current target is to try and have new drop structures in place to move some water into Fresno this fall.

The Milk River Project’s purpose is to provide irrigation water for approximately 110,000 acres along with municipal water for several Hi-Line communities. Happily, fish, wildlife and outdoor recreation coincidentally benefit from this water as well.  Directly and indirectly the water provides significant economic benefits to Hi-Line communities.

As the Milk River flows almost 700 miles from its headwaters in Glacier National Park to its confluence with the Missouri River near Nashua, crossing parts of Montana and Alberta on the way, it also fills two important reservoirs in northern Montana: Fresno and Nelson. The reservoirs’ primary function is water storage for irrigation and flood control. Currently, Fresno is down approximately 9 feet and Nelson is down 2 feet. As water demands increase this summer, drawdowns to both reservoirs will occur.

Fresno Reservoir is expected to see a significant drawdown and is projected to be 24 feet below full pool by Aug. 15. Studies have shown that reservoir levels 20 feet below full pool for extended periods of time, especially into winter, can be extremely detrimental to the fish community.

Nelson Reservoir is projected to drop about 5 feet by mid-August, which will have no significant impact on the fishery. Nelson fisheries tend to see more significant impacts if the reservoir falls to more than 12 feet below full pool.

Impacts to the fisheries of the Milk River itself will be determined by how long the river sections are dry or see limited flow. The river above Fresno could see significant impacts as Canadian irrigators will be utilizing their Milk River natural flow water right. Overall, Milk River impacts will be mitigated if there are good late summer and fall rain and if the repairs to the St. Mary Canal water structures are completed by fall.

For more information on the Fresno or Nelson reservoir fisheries, please contact biologist Cody Nagel in Havre at 406-265-6177 or email cnagel@mt.gov.