Managers of the Columbia River hydropower system began tinkering with in-stream flow rates last week in an effort to encourage spring chinook migration. It’s good to see new creative thinking about how to enhance river conditions.

The four-hour slowdown in river flows Thursday didn’t seem to have a dramatic immediate impact on migration, which has been lagging far below preseason expectations. However, chinook counts rose to nearly 6,000 at Bonneville by Sunday, after being as low as 1,746 May 2. It’s probably too soon to draw a direct cause-and-effect conclusion, but something suddenly started moving the fish inland.

The preseason forecast was for 314,200 mature chinook to reach the mouth of the Columbia, which would be the fourth highest since 1980. As in other recent years, this prediction was largely based on the heavy early returns of immature “jack” salmon last season. Unfortunately, in the past several years jacks have been proving to be a fickle forecasting mechanism.

As a general rule based on the 10-year average, by May 6, half of the spring run should have passed Bonneville, 146 miles from the mouth. If that turns out to be true this year, the total run would only be an abysmally bad 90,000 or so. No one really expects it to be anywhere near that awful, but it still warrants the step taken last week by the U.S. Army Corps of Engineers.

We often think of salmon waiting for spring freshets, or pulses of floodwater, to signal it is time to move upstream. But this year, the theory is that high flows are discouraging the chinook, which may be loitering in the estuary and ocean. Last Thursday, water flows through Bonneville started and ended the day at 429,000 cubic feet per second, but were lowered to 333,000 during the hold-back.

All this goes to show how little we know about what drives chinook behavior and abundance. With luck, the strategy of lowering in-stream flows in some circumstances will prove to be a useful management tool.

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