Despite a dozen triple-digit days recently that had the potential to trigger fatal fish diseases, this summer’s Rogue River salmon migration has gone quite swimmingly, thanks to plenty of stored reservoir water.

Lost Creek Lake water releases have buoyed Rogue flows enough that it washed away biologists’ fears that spring chinook salmon en route to the upper Rogue would die from warmwater-borne diseases.

The favorable fish conditions have triggered the best spring chinook migration to Cole Rivers Hatchery in Trail since 2017.

And that comes despite a dozen days of air temperatures of 100 degrees or above in Medford, according to the National Weather Service.

“Without the extra (stored) water we ended up getting, we would have had dead fish,” said Pete Samarin, an Oregon Department of Fish and Wildlife biologist who helps craft Rogue Basin water-release plans.

“To me, that’s pretty amazing,” Samarin said.

The next big test begins in the coming week when the agencies reset water-release plans for the next wave of Rogue salmon — fall chinook.

At ODFW’s behest, the U.S. Army Corps of Engineers will begin increasing water releases from Lost Creek Lake by 20% to help the Rogue’s fall chinook to fare as well as the basin’s spring chinook.

The buoyed releases were scheduled to begin Thursday, but Samarin called off the move.

Cooler air temperatures and evidence from ODFW fish-netting surveys in the lower Rogue show that chinook weren’t yet leaving the cool confines of the Rogue Bay, so Samarin will revisit the planned releases early next week.

Just a five-day delay in jacked-up flows gives Samarin confidence that the Rogue will again sidestep a salmon die-off.

“We’re not in the business of wasting water,” Samarin said. “We’ll wait until Monday.”

When reservoir levels were dangerously low in March, biologists were expecting anywhere from a third to half of this year’s wild fall chinook run to die from warm-water diseases.

But the wet spring filled Lost Creek and its sister reservoir, Applegate Lake, and allowed biologists to craft Rogue flow regimens to stave off disease outbreaks, which have not been seen here in more than two decades.

Biologists have linked ambient air temperatures to warming Rogue waters in the Agness area, where spring and fall chinook must navigate the lower Rogue canyon to reach the cooler confines of the upper Rogue.

Hot weather can raise water temperatures high enough to trigger outbreaks of natural diseases such as columnaris, which attacks the gills of chinook and can suffocate them en masse.

This spring, it looked like the first columnaris outbreak since 2001 was imminent.

“We were expecting dead fish in July, and more dead fish next week,” Samarin said.

But a funny thing happened on the way to this summer’s water pity-party.

Spring rain — and near-record snowfall at Crater Lake — pushed Lost Creek Lake to nearly full and raised outflow forecasts well above average, all but guaranteeing a safer summer on the river for salmon and people.

The best disease deterrent is to keep summer flows artificially high, which keeps temperatures down. And this year, that’s taken some work.

Since June 25, the National Weather Service in Medford has logged 12 days of triple-digit weather, including six straight from July 24 through July 29, said meteorologist Marc Spilde.

The hot stretch peaked July 29 when the thermometer hit 114 degrees in Medford, Spilde said.

Though there’s no way to predict with confidence the temperatures here later this month, odds are Rogue Valley residents — including its wild salmon — have seen the worst of the year.

As days shorten and the sun beats down on the region for fewer and fewer minutes each day, “the likelihood of 100 degrees goes down,” Spilde said.

“It’s much, much harder because the days are that much shorter,” Spilde said.

That said, triple digits are not unheard of in late August and even into late September, Spilde said.

The Weather Service has logged super-hot days in Medford as late as Sept. 27, when the temps peaked at 102 degrees in 1963 and again in 2003, Weather Service records show.

Mark Freeman covers the environment for the Mail Tribune. Reach him at 541-776-4470 or email him at mfreeman@rosebudmedia.com.

See video story on Oregon Outdoors: https://www.mailtribune.com/oregon-outdoors/2022/08/11/oregon-outdoors-upper-rogue-steelhead/