South Fork Boise River flows downstream of Anderson Ranch Dam
The Bureau of Reclamation began turning down the dial on the boating flows after the end of the Labor Day weekend.
By the end of Tuesday September 4th flows dropped from 1,850 cfs to 900 cfs. On Wednesday the 5th the flows were at 600 cfs.
Summer boating flows lasted longer and were slightly higher than the typical 1,600 cfs. In fact, the 1,800 cfs flow seems to be a “new normal.”
So why did flows stay higher through Labor Day? Water flows from the storage dams in the Boise River system had both a normal component as well as one aspect not normal in 2012. This summer there was a big pull on stored water from Anderson Ranch Dam. The graph below shows the blue line (this year) dipping well below the long-term average for water in the reservoir. And that was after starting at a record level. The flows stayed high until Labor Day becuase of the need to deliver stored water to the downstream irrigation demand.
Water in Anderson Ranch Reservoir - 2012 compared to 2011 and the long-term average.
So the water flows from Anderson Ranch have pulled the reservoir down to a below-average level. But when you examine the total amount of storeage water relased this year the graph below shows the amount of water in the entire system of storage reservoirs remains above the long-term average.
Total water stored behind Boise River system reservoirs - 2012 compared to 2011 and the long-term average.
The total amount of stored water released this year was no more than average (thus the parallel lines). It was largely a matter of proportionally more water being in Anderson Ranch Dam that needed to be sent downstream in order to meet water demand. Irrigation is one factor. Another is timing. Water managers keep Lucky Peak full until Labor Day. Then they draft it through September for irrigation and in the winter for streamflow maintenance (which helps preserve the wild trout fishery through Boise).
So with three weeks to go we will end the water year with more water than average in the entire system but less than average water in Anderson Ranch Reservoir.
