It’s flip-flop time again in the Yakima River basin, which is good news for rapids rafters, fly-fishers and salmon.

The federal Bureau of Reclamation announced Friday it would begin its annual manipulation of reservoir flows — known as the “flip-flop” — this week. The flip-flop, which reduces flow in the upper Yakima, Cle Elum and Bumping rivers while increasing flow in the Tieton and Naches rivers, is expected to begin Tuesday.

According to the bureau, the aim is to help chinook salmon spawn and incubate while stabilizing irrigation water supply.

“This annual flip-flop operation maintains relatively low, more natural flows, which are important for chinook salmon spawning in the upper Yakima, Cle Elum, and Bumping rivers,” Chuck Garner, Yakima Project River Operations supervisor, said in a news release. “It also allows us to reduce impacts to irrigation water supplies because lower flow releases improve reservoir storage for the coming season.”

It’s been done every year since 1981, when it was created in response to a 1980 federal court ruling by Judge Justin Quackenbush that ordered Reclamation to meet the needs of water users “through more efficient or less extensive use of project waters or by modification of project operations or facilities so as to have less impact on the fisheries resource.”

By Sept. 10 flows out of the Cle Elum Reservoir will be reduced to about 180 cubic feet per second, down from a July 10 high of 3,354 — or about 1,346 gallons per second, down from 25,090. Meanwhile flows from the Rimrock Reservoir will increase from between 800 and 1,500 cubic feet (between 5,984 gallons and 11,220 gallons) per second during the Labor Day weekend to between 1,700 and 2,400 (12,716 and 17,952) by mid-September.

The primary goal is to allow salmon to spawn at lower, more natural water levels. But there have been a couple of prominent side effects that rafters and anglers have come to appreciate over the past 38 years. The change in flows creates outstanding whitewater rafting on a 12-mile stretch of the Tieton River, turning the normally tranquil waters into Class 3 rapids for a month. It also makes for much better fishing on the lower Yakima, with lower water levels making wading easier and giving wily trout fewer hiding places.

Officials caution people recreating or working along Yakima basin rivers to be careful, stay away from the areas where spillway water flows into the river, portage around buoys and stay out of dangerously turbulent flows.

Reach Pat Muir at pmuir@yakimaherald.com.