Friday, July 20, 2007

Low Flows --> Hot Water: Big Hole Watershed Committee still failing

When flows drop in the Big Hole River, the river's fluvial Arctic grayling find themself in hot water. Confluence Consulting pointed out this correlation between flow and water temperature to the Big Hole Watershed Committee years ago. The problem is exacerbated by the lack of riparian cover (i.e. stream shade) and by the shallowing/widening of the river channel as overgrazed/devegetated banks collapse into the river.

Even though flows have rebounded slightly (and probably temporarily, as ranchers close irrigation ditches during haying), the water is too warm for grayling survival.

Bottom line: Big Hole River temperatures are daily exceeding 25 deg C, or 77 deg F. According to fisheries biologists, 26 deg C is the "lethal level" for salmonids. At this temperature, 50% of the fish die. High temperatures are especially hard on young fish. To avoid killing salmonids, fisheries biologists recommend that temperature never exceed 20 deg C--even for a short amount of time.

For the data on current upper Big Hole River flows and temperatures, see

For more info on temperature stress in salmonids, see D. McCullough (1999), "A review and synthesis of alterations to the water temperature regime on freshwater life stages of salmonids" EPA 910-R-99-010; and S.C. Lohr et al (1996), "High-temperature tolerances of fluvial Arctic grayling and comparisons with summer river temperatures of the Big Hole River, Montana" Transactions of the American Fisheries Society 125: 933-939.

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