The heat wave that settled over the region in early July apparently caused a fish kill on the Middle Fork of the John Day River.
Jeff Neal, assistant district fish biologist for the Oregon Department of Fish and Wildlife, said the agency investigated after receiving a report of dead fish on a stretch of the river.
On July 6, officials found 118 dead fish in the shallows of the river between Beaver Creek and the Phipps Meadow area, a span of about 20 miles. About 37 live fish were counted in that stretch, Neal said.
Neal said the fish were spring Chinook salmon, native fish that migrate to that reach in June and remain there until spawning time in September.
Neal said it's not uncommon to lose a dozen or so fish during the summer there, but nothing of this magnitude.
Fish that are weakened by parasites or disease commonly are more susceptible to the ill effects of temperature changes, he said.The ODFW has sent samples to its lab to determine any parasite or disease problems that may have been a factor in the incident. Results are not yet available.
"Barring the testing, we're pretty sure this was caused by a 20-degree temperature change," he said.
The water temperature in that part of the river typically will rise from the low 60s to the 80s during the summer months, Neal said. In this case, biologists believe the water temperature jumped 20 degrees in a period of just three days.
Such a sudden increase can cause "temperature shock," said Neal.
From July 6 and July 12, agency workers also surveyed other waterways including Vinegar Creek, Granite Creek and Clear Creek, and found no kills.
Neal said they found a few dead fish on the North Fork John Day River near Highway 395, but it was not an unusual amount for this time of year. They also counted numerous live fish in that area, he said.
He noted that the North Fork, unlike the Middle Fork, has some 8- to 10-foot-deep holes where fish can still find cool water.
"The weather pattern really caught them in a bad position up there," said Neal.
The air temperature during the first week in July topped 100 degrees on repeated days.