ENNIS, Mont. (AP) — Wildlife workers and volunteers scrambled Wednesday to save fish stranded by an abrupt drop in water levels on a Montana river that’s renowned among anglers.

A malfunction on the Hebgen Dam caused flows into the Madison River to plummet early Tuesday, Montana Fish, Wildlife and Parks said. That left some side channels cut off from the main river and outfitters report that some fish already have died.

Many more fish are at risk as the side channels dry out, fly fishing shop owner Kelly Galloup said. Volunteers on Tuesday began scooping fish out of the channels and returning them to the main stem of the river.

Northwestern Energy operates the dam. The utility says river flows may not be fully restored for several days.

The Madison flows out of Yellowstone National Park in Wyoming and is a highly-regarded “blue-ribbon” trout fishery.

State officials said the damage inflicted on the fishery was still unclear.

Galloup said many bigger trout likely moved to deeper pools as water levels dropped. But he expects to see a significant impact on fish born last year that were too small to escape the sudden drop in flows.

“We scooped yesterday all day on the upper river,” Galloup said. “We’re just concentrating on the side channels, trying to rescue whatever we can find.”