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Uncertain Water Supply Forecast Results In Cautious Dworshak Operation Changes
Posted on Friday, January 12, 2018 (PST)

Heavy rain in the North Fork Clearwater River basin expected this week will cause inflows to the Dworshak Reservoir to rise to 4,500 cubic feet per second late in the week and will reach as high as 10 kcfs by the weekend.


With a target elevation for the reservoir set by the U.S. Army Corps of Engineers at 1,530.5 feet at the end of January, a rise of 10 feet, Steve Hall of the Corps said that flows from Dworshak Dam will have to also rise to 2.4 kcfs from the current flow of 1.8 kcfs, a flow measured as of 9 am, Wednesday, Jan. 10. The change to outflow was determined at the interagency Technical Management Team meeting Wednesday.


All of the outflow will be through Unit 1, Hall said, the only turbine of three at the dam that continues to operate. Full load for the generator results in a flow of 2.4 kcfs.


Salmon managers at TMT had hoped for a change in flows to 2.5 kcfs, but Hall said the remaining 100 cubic feet per second of flow would have to be through spill, at least for now.


The Corps took Unit 2 offline Jan. 9 to be serviced, expecting it to come back online Feb. 8. The dam’s largest generator of three, Unit 3, has been out of service for repair for more than a year and won’t be back in service until summer, leaving just Unit 1 to pass water.


The Corps announced last July that Dworshak’s largest turbine would be out of service for nearly another year, until July 1, 2018. Initially, the Unit-3 overhaul was to be completed July 15, 2017. Without the turbine, which accounts for more than half the dam’s power output, the Corps must at times spill more water for both summer and winter operations. Spill can result in higher than allowed levels of total dissolved gas in the North Fork Clearwater River. The Unit 2 outage exacerbates the situation.


In setting the Jan. 31 reservoir elevation, Hall said, the uncertainty is in the water supply forecasts.


The Corps’ water supply forecast ( in the North Fork basin for the April through July period is 2.94 million acre feet, 109 percent of normal for the long term average (1929 – 2008) and 121 percent of normal for the more recent 30-year average (1981 – 2010), Hall said.


However, the River Forecast Center is forecasting 2.648 MAF, 103 percent of the 30-year average, April through September.


The difference is in the HQSI Cumulative Precipitation factor, which the Corps adds into the forecast, but the River Forecast Center does not, he said.


“That’s included to give us an indicator of soil moisture,” Hall said. “For now, it may have too much influence on the forecast. Without it, we would be at 2.5 MAF.”


The end of January reservoir elevation could have been set at 1,533 to 1,534 feet if the Corps used the 2.5 MAF forecast, he said. But this time of year, weather and inflows are unstable and change constantly. “1,520.5 feet is conservative.”


He added that the longer range weather forecast for the basin is for wet and warm conditions.


“We’ll likely go into April with a less than normal low elevation snowpack and, maybe, above normal high elevation snowpack,” he said. “That could mean a later runoff.”


Also see:


-- CBB, July 21, 2017, “Dworshak’s Largest Turbine Out Another Year; Poses Challenges For Salmon Management,”


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