[7] Little Goose Spill Changed After Test Blocks Adult Migration

The Corps of Engineers changed the spill pattern at Little Goose dam last month after fish watchers noticed a drop in the number of chinook between Little Goose and Lower Granite dam. The change improved the adult migration almost immediately.

Late last month, the Corp was testing spill conditions at the dam after installing a removable spillway weir, scheduled for use during the 2008 migration season.

On May 25, daily adult counts at Little Goose declined noticeably, down to 32 fish by May 27. Spring chinook backed up below the dam until the 31st, when the original spill pattern was restored and more than 4,600 fish rushed past the dam over the next two days.

The fish moved quickly once the "bulk" spill pattern requested by NOAA Fisheries was restored to a flat pattern around 4 p.m. May 31. Twenty-six adults were counted between 6 and 7 p.m., but 798 adult chinook were tallied at the fish ladder in the following hour.

The significant delay experienced by the fish raises questions over potential tradeoffs between benefits of a removable spillway weir for juvenile migrants and adverse effects on adult migrants.

Corps researcher Rock Peters said the agency needs to go back to its hydraulic model to develop the best spillway pattern. "We're confident we can do it."

Peters said when flows decline by just a little, especially in the powerhouse of facilities like Goose, hydraulic conditions in the spillway can change a lot. -B. R.

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