Tuesday, June 19, 2007

Big Hole River grayling survival index (19.June)

Today's grayling survival index is 28, based on a flow at Wisdom (see http://waterdata.usgs.gov/mt/nwis/uv?06024450) of 88 cfs (cubic feet per second).

0 = minimum survival; 100 = maximum survival

Flows in the upper river are dropping like a rock, despite wetter-than-normal weather. Cattle are standing in water on the irrigated hay meadows while grayling are losing out.

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. Today, 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 = (88 - 60) = 28.

No comments: