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May Bighorn River Flow Update

Low Snowpack, Reservoir Levels Lead to River Decreases

Contrary to recent years past, precipitation throughout the Bighorn Basin is 50 to 75 percent below normal this May, when precipitation is typically highest on average. In addition, snowpack levels are lower than average and snowmelt runoff forecasts continue to decline throughout the Bighorn River Basin resulting in higher irrigation demands and lower inflows into Bighorn Reservoir. To maintain accordance with reservoir draw down rates and inflows, the Bureau of Reclamation is dropping Bighorn river flows to 2,000 cfs this week.

Dropping flows during the Rainbow trout spawn is not ideal. Side channels can become disconnected and established redds can dry up, impairing recruitment of young trout into the main population. Recommendations from MFWP and the BHRA have been to maintain the river at 2500 cfs as a preferred fishery flow to support both fall spawning Brown trout and spring spawning Rainbow trout. The absolute minimum fishery flow is between 1500 cfs and 2000 cfs, which still offers recreational benefits to anglers. Fortunately, Bighorn trout are particularly excellent main channel spawners.

However, MFWP population trend data and a USGS study of western tailwater fisheries indicate lower and consistent flows in the spring can have a positive effect for rainbow trout populations particularly spawning success. This spring the Brown trout hatch should have been successful with good winter flows for spawning and recruitment. If successful, this would provide two upcoming good years of Brown trout production. The 2019 brown trout are currently growing well and about 7 inches. There is a gap with few fish from 7 to 15 inches with some older fish still around and very heavy for their length with fish into the 20-inch range. Rainbows have been hanging on with some reproduction, but the rainbow trout fishery is mostly larger fish. The Rainbow Trout spawn is occurring now and will last for several more weeks. These lower flows, while not entirely desirable may provide conditions for rainbows to have a better spawning season than the past several years.


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