Friday, August 8, 2008

Big Hole Watershed Committee: Big Hole River grayling survival threatened

Today's Grayling Survival Index = -5
A negative index means there is not enough water to insure minimal survival. Too much water is being used by ranchers to grow hay.

This has been a good water year. Not an exceptional year such as 2006-07, but it certainly hits the long-term average. Spring was significantly wetter and cooler than average, which meant that there was far more snowpack to feed summer flows.

Despite this, the Big Hole River in the Wisdom reach -- a section of the river that is critical for fluvial Arctic grayling survival -- has dipped below 60 cubic feet per second. 60 cfs is the lower wetted perimeter, the minium flow needed (according to fisheries biologists) to maintain grayling populations. Below 60 cfs, mortality becomes high. Fish die.

What is the Big Hole Watershed Committee's problem, if even in a good water year, it cannot maintain minimum flows for our endangered fluvial Arctic grayling?

You can see from the hydrograph (below) that haying ended lasted week, and as the irrigation ditches were opened back up, river flows sank. Yesterday's big rainstorm helped a little, but not enough. True conservation is the only thing that will help.

Fact is, more and more hay meadows and pastures have been irrigated in recent years. Furthermore, hay meadow irrigation has increasingly shifted over to pasture irrigation. Hay irrigation largely ceases after the hay is cut. Pasture irrigation, however, requires water every day, all summer long. When you add global warming to this equation, it means that fluvial Arctic grayling in the Big Hole River are probably doomed.

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EXPLANATION OF THE GRAYLING SURVIVAL INDEX:Less than 0 means a loss of grayling; 0 means minimum sustainable level; 100 means maximum survival. For this blog, the flow of the Big Hole River at Wisdom, Montana, provides an index of how well the Big Hole Watershed Committee is doing in its efforts toward water conservation and grayling restoration.

The Watershed Committee is doing a lousy job. According to fisheries biologists, the upper wetted perimeter at Wisdom is 160 cfs (cubic feet per second). The lower wetted perimeter is 60 cfs. The minimum "survival flow" is 20 cfs. This "survival flow" is not scientifically based, but it is the flow level that fisheries biologists "feel" allows grayling the ability to escape warm water and to seek cold water refugia (tributary streams).

Criteria for grayling survival index ratings:
* At 160 cfs, grayling recruitment and survival is rated at 100. At this level, the streambed is fully wetted or bank-full. This level allows grayling the maximum use of stream "pasture" for foraging, hiding, spawning, etc. This level maximizes grayling recovery.
* At 60 cfs, grayling recruitment and survival is rated at 0. At this level, the streambed is minimally wetted. This level allows grayling to maintain their population. This level does not aid in the recovery or increase of the grayling population. At best, this level might maintain the current population level. Below this level, the streambed rapidly becomes dry and thus barren of aquatic life.
* At 20 cfs, grayling recruitment and survival will be rated at -100. At this level, some grayling will be able to move to cold water refugia (tributaries), but many will perish due to lack of cover, exposure to predators (such as pelicans), and high water temperatures.

For stream flows above 60 cfs, the grayling survival index = y = mx + b = x - 60. For stream flows below 60 cfs, the grayling survival index = y = mx + b = 2.5x - 150. Thus, today's grayling survival index = 2.5(19) - 150) = -102.5.

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