Hoping to prevent water temperatures at Lower Granite Dam from rising above 68 degrees Fahrenheit, the U.S. Army Corps of Engineers has been spilling water at Dworshak Dam on the North Fork Clearwater River, which holds back significantly cooler water compared with the lower Snake River.

On July 17, the water temperature at Dworshak Dam was about 43 F compared with 67 F at Lower Granite.

Fish managers say keeping water temperatures below 68 F in the lower Snake River benefits juvenile fall Chinook emigration and rearing, and also helps adult sockeye, spring/summer Chinook, steelhead and fall Chinook migration. The temperature limit is part of Dworshak Dam's biological opinion to aid fish survival.

Jon Roberts, a Corps reservoir regulator for the Walla Walla District, told the Columbia River Technical Management Team (TMT) on July 17 that dam operators would begin discharging 13,000 cfs on July 18, including operating at full powerhouse of 9,500 cfs. Operators expected to increase discharges at Dworshak to 14,500 cfs to prepare for even warmer weather next week.

"We're going to push as much water as we can on Friday and Saturday. That's the last few days of the cool-down" and will give the released water time to reach Lower Granite by Monday, when warmer weather is predicted, he told TMT members.

He said total dissolved gas is expected to remain below 110 percent, and operators will reduce spill as TDG approaches that limit.

Roberts said the Corps hopes to taper off spill as quickly as possible in order to save some of the cooler water for later in the season.

Even with the cool water releases, Corps models show that water temperature at lower Granite could exceed 68 degrees after about eight days, and continue for at least a few days, Roberts noted. "Those models could change as weather forecasts are updated and as we get closer to the actual dates."

Current weather forecasts predict air temperatures to jump as much as 20 degrees early next week, he noted. The National Weather Service shows a predicted high of 81 F on July 19 at Orofino, Idaho, which is close to Dworshak Dam. Highs are forecast to reach 99 F on July 21 and 101 F on July 22, the Weather Service predicts.

K.C. Mehaffey covers fish issues for Clearing Up, and is editor of the NW Fishletter. She joined the NewsData writing team in February 2018. From lawsuits to scientific studies, she is enjoying the deep dive into the Columbia Basin's many fish topics.