The flow regime in 2008 on the South Fork Boise departs from the standard practice of increasing flows to 1,600 cfs in late May and instead this year the flows have remained at 600 cfs through the month of June. As the upper graph shows, compared to last year and to “average” flow releases, we are well below flow levels even though snowpack is average. This is not normal. Why a difference?
Well, 2008 is one of those years where average snowpack follows a dry year where the Boise River reservoir system was largely drained (see second graph). Following the 2007 drought, where the green line in the second graph dropped to about 120,000 acre feet, water managers want to preserve all water use options by storing water as high up in the system as possible. In the spring of 2008 they also faced a substantial flow from the Middle and North Fork Boise Rivers that quickly filled Arrowrock and Lucky Peak reservoirs. The cooler than normal spring also controlled the amount of snowmelt so except for a ten day period with a large flow spike in the Lower Boise River, run-off for this year has to be considered among the more optimal for a water conservation perspective.
In the shuffle it was the flows from Anderson Ranch Dam that have been less than normal. The desire to fill Anderson Ranch from it’s rather empty condition as well as the need to let the Middle Fork Boise flows fill the lower reservoirs combined to make for a unique 600 cfs through June.
As of late June Anderson Ranch Reservoir is at 97 percent full and inflows from the North and Middle Fork Boise Rivers are still providing enough water for lower Boise River irrigation deliveries. But within a week more water will be called for the canals and the reservoirs will provide it. Flows from Anderson Ranch are to be increaed following July 4. Let’s hope the timing of the flow change does not mess with the fish sampling effort on July 12.
