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How to Measure Dissolved Gas Levels at Bonneville Discussed by CBB Staff Columbia Basin Bulletin - April 9, 2004

A change proposed by salmon managers of where total dissolved gas levels would be measured could change the amount of water voluntarily spilled at Bonneville Dam.

Long a point of contention at Water Quality Team meetings, but never resolved, salmon managers broke away from WQT and sent their own letter directly to the U.S. Army Corps of Engineers asking the Corps to use a TDG monitoring device already located in the dam's tailrace when making in-season decisions about the level of spilled water.

The Corps now uses the Camas/Washougal monitoring station 25 miles downstream in Washington when making day to day adjustments to spill.

When river TDG levels rise above 115 percent at the Camas/Washougal monitoring station, the Corps reduces the amount of water spilled to bring TDG within allowed limits and vice versa, spill may be increased when TDG is below 115 percent.

However, salmon managers said that monitoring station is too far downstream to make quick decisions about dam operations.

"The tailrace monitor gives a good measurement of dissolved gas," said Mark Schneider of NOAA Fisheries at last week's Implementation Team meeting.

The Camas/Washougal monitoring station is the only such TDG monitor specifically mentioned in the NOAA Fisheries 2000 biological opinion of the federal hydro system, and it has been a problem since it was installed, said the letter to the Corps' Jim Adams from the salmon managers. They go on to say there is "no compelling reason to retain Camas/Washougal as a TDG monitoring station."

Schneider said the choice is whether to measure TDG at the point of spill or to measure spill further downstream. Environmental factors, such as wind, barometric pressure and sunlight, alter the amount of dissolved gas in the river between the dam and the Camas/Washougal station. However, it's unclear whether the change in monitoring locations would reduce or increase the amount of water spilled. While Schneider said it may reduce spill, the Corps thinks otherwise.

"I've been told the Camas station tends to constrain spill more often than would the tailrace monitor," said Jim Athearn of the Corps.

Whatever the decision about the monitor location, the Bonneville Power Administration wants more study before the Corps agrees to abandon the Camas/Washougal site and before it begins to use the spillway as a monitoring site for in-season decisions.

"I'd like to see the effects on spill and survival and have NMFS put that in the context of performance standards," said Bill Maslen of BPA.

The Corps also maintains other monitoring stations in forebays at each Columbia River Basin dam in order to measure the loading of dissolved gas in the river.