crowded boats at Oregon City

Boats crowd the Willamette River between Gladstone and Willamette Falls during spring chinook salmon season. Warm water temperatures are killing fish in the river before they have a chance to spawn.

(Bill Monroe)

This post has been updated. Original story: Warm waters kill spring chinook in Willamette River

Oregon's worsening drought has taken its first official victims.

Abnormally warm water temperatures in the Willamette River are killing spring Chinook as they swim upstream to spawn.

Anglers on Wednesday began reporting dead salmon floating downstream and washing ashore near Willamette Falls, casualties of water too warm for their survival.

Temperatures in the lower Willamette have spiked amid the recent heat wave, rising well above 70 degrees, said Rick Hargrave, a spokesman for the Oregon Department of Fish and Wildlife.

"The Willamette gets warm, but we wouldn't normally be seeing temperatures like this until July," Hargrave said.

The highest temperature was recorded near Newberg, where the water reached 75 degrees.

That's well into the danger zone for salmon, which prefer water in the 50s and become stressed once the mercury tops 60 degrees. Each degree of additional warmth increases their likelihood of death.

It's also a sharp increase from this time last year, when a thermometer at the Willamette Falls fish counting station registered 63-degree waters.

"Just a couple of degrees can make a big difference in the aquatic world," said Rick Swart, a northwest regional spokesman for ODFW.

State fish biologists were on the Willamette and Clackamas rivers Thursday surveying the die-off. During an hour of preliminary investigation Wednesday night, a biologist counted more than 50 carcasses.

A poster on the website ifish.net reported seeing at least 30 of the dead fish floating down the river or lying on the banks Wednesday evening near Oregon City's Clackamette Park, located at the confluence of the two rivers.

"I've never seen this many before," the poster wrote. Other commenters on the online forum echoed the author's concerns.

This is Oregon's first temperature-related fish kill of 2015, but more are likely as Oregon's drought continues to affect waterways.

With virtually no snowpack left in most Oregon mountain ranges, stream flows across the state are far below normal. The shallow, slower-moving waters warm quickly to create treacherous conditions for fish.

In addition to creating dangerously warm conditions, low water levels can leave fish stranded in deep pools with flows too low to permit their escape.

Already, the drought's impact on waterways has forced ODFW to alter its management practices. The agency announced earlier this month that it would release trophy trout from the Nehalem Hatchery months earlier than planned. Flows at the hatchery are too low to raise the fish to full-size, they reasoned.

Then, last week, the agency decided to let anglers fish from boats on a section of the Sandy River where fishermen are normally confined to the riverbank. The reason: The lowest flows in more than 20 years have restricted fish movement, making it difficult to for anglers to catch anything from the banks.

Swart said district biologists throughout the state have been asked to identify drought-related issues that could arise in their districts and detail their response plans.

"If the warm water conditions persist, there will be more," Swart said. "Today we're talking about the Willamette, tomorrow we might be talking about the Rogue."

Fortunately, he said, this week's die-off has made only a small dent in an otherwise successful spring Chinook run. More than 51,000 fish have crossed at Willamette Falls, well above the 50-year average of 41,000 fish.

-- Kelly House

khouse@oregonian.com

503-221-8178

@Kelly_M_House