[4] High Flows, Debris, Affect Fish Passage At Dams

Dam operators and fish managers agreed last week to raise screens at Bonneville Dam's second powerhouse to reduce injury to juvenile fish caused by debris pinned to the screens in front of the turbines. The screens are designed to guide fish into the dam's smolt bypass system.

Snow melt from the recent warm weather had increased flows at Bonneville to about 400 kcfs and flows climbed even higher by the end of last week. The high flows were likely responsible for the levels of trash at the dam.

By May 20, about 19 percent of the smolts examined showed descaling, some very severe. Fish managers made a formal request to run turbines at the low end of 1-percent efficiency to reduce powerhouse flows and boost flows to a 125-percent total dissolved-gas limit.

Managers decided that cleaning the screens would take too long, compared to the immediate benefits of their temporary removal. They also decided to operate several turbines at PH II at the low end of 1-percent efficiency, and others at the mid-point of 1-percent efficiency. This was expected to slightly reduce flows through the powerhouse, but the increase in spill from the change was expected to raise TDG levels by only 3 to 5 percent.

Meanwhile, TDG gas levels spiked at lower Snake dams from peak flows that pushed toward 200 kcfs by last Thursday. Gas levels in the tailrace of Lower Granite Dam nearly reached 134 percent, as more than half the river went over the spillway, well above the 120-percent tailrace gas cap.

This spring, more than 6 million spring chinook and steelhead smolts have been barged from lower Snake River dams. Numbers of juvenile spring chinook are tailing off fast in the Snake from the peak day on May 8 at Lower Granite Dam, when around half a million young salmon passed the dam.

After flows peaked, dam managers decided to screen one turbine intake at Bonneville Dam before the next pulse of Snake River smolts show up.-B. R.

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