Heat wave kills fish on Priest River, highlighting the need for a cold-water bypass

Last week’s heat wave killed some fish on the Priest River, according to Idaho Fish and Game biologists and reports from concerned residents and anglers.

While the timing of those deaths was earlier than normal, the fact that they died isn’t all the unusual, IDFG regional fisheries manager Andy Dux said.

“Every year we have some issues with water temps getting really warm and at times above the limit for trout,” Dux said . “The real issue for this year is that it’s happened earlier than we usually see and it happened so suddenly as well.”

Dux said he heard about 40 to 50 fish dying in one location on the river, as well as reports of smaller die-offs elsewhere. Most of the dead fish reported were mountain whitefish along with some westslope cutthroat trout, Dux said. There aren’t many lake trout living in the Priest River.

“I don’t have temperature data from the location where the fish kill occurred,” Dux said in an email. “However, based on temperatures we observed in other streams last week, the river likely reached temperatures well above 70 degrees. The lethal temperature limit varies for each fish species, but typically trout and whitefish struggle when they have prolonged exposure to temperatures over 70 degrees.”

The deaths shouldn’t have a meaningful impact on the river’s fish populations, Dux said, but that could change if temperatures remain high throughout the summer.

“If we have a ridiculously hot summer … we could see more of this,” he said.

It underscores one of the problems that a warming climate presents, particularly for rivers that don’t have a steady influx of cold water year round.

Unlike the Spokane River, the Priest River has no cold-water influx keeping it cool during the heat of summer. Instead, sun-warmed water from the top of Priest Lake flows into the river. During the summer, this means the water often gets too warm for cold-loving species like trout.

That’s why IDFG is considering constructing a cold-water bypass that would pump colder water from the depths of Priest Lake over the dam and into the Priest River with the aim of creating a trout fishery.

“This is a prime example of why that idea is being considered,” Dux said. “Because it would help.”

The proposal has met resistance in the past, with some worrying that the bypass would impact summer recreation and the ecology of Priest Lake.

IDFG has contracted with a private consultant to assess whether the bypass would have any unintended ecological impacts on the lake or river. That report should be finalized and published by the end of the summer, Dux said.

Meanwhile, for residents and anglers, last week’s deaths have sparked concern.

Betty Gardner has lived on Priest Lake for 42 years. On June 29, she saw five dead fish floating in Priest River, a day after record setting heat baked the region.

“On Wednesday there were 21 dead white fish and one large rainbow trout, more on Thursday and Friday,” she said in an email.

She then checked the water temperature on June 30. It was 83 degrees.

“The United Sates Forest Service predicts that the weather we have seen for the last few years is the new normal,” she said in the email.

“We can expect higher summer temperatures, less snowpack, less rainfall, more droughts and more forest fires. There will be more fish kills.”