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Maintaining Court-Ordered Spill At Columbia/Snake Dams Without Exceeding Gas Caps Proves Challenging
Posted on Friday, May 04, 2018 (PST)

Maintaining court-ordered spring spill at maximum state-allowed total dissolved gas levels, known as gas caps, at eight federal dams continues to challenge the U.S. Army Corps of Engineers.


A three-judge panel of the Ninth Circuit Court of Appeals ruled April 2 in favor of an April 2017 U.S. District Court injunction allowing more spring spill at four lower Snake and four lower Columbia river dams. With the decision, spill to the gas cap began April 3 at lower Snake River dams and at lower Columbia River dams April 10. The additional spill through June 15 is designed to aid migrating juvenile salmon and steelhead.


Nearly every day since the Corps began the new spill regime at the dams, TDG levels have exceeded state water quality standards – 120 percent in the dam’s tailrace and 115 percent in the downstream dam’s forebay. Staying at those levels without at some point during the day exceeding them is difficult, Dan Turner of the Corps’ River Control Center told the interagency Technical Management Team at its meeting Wednesday, May 2.


Percentage caps on total dissolved gas (caused when spill plunges into the river) are intended to protect young fish from gas bubble trauma during spill.


The Corps has developed levels of spill – spill targets – that it believes would result in the allowed TDG levels. Those targets change as flow increases or decreases. For example, the spill target April 20 at Lower Granite Dam was 45,000 cubic feet per second of spill. That created TDG of 119 percent in the dam’s tailrace (within TDG limits), but it was resulting in 116 percent TDG in Little Goose Dam’s forebay, which exceeds the 115 percent gas cap limit. These exceedances extended through April 28 until river flows began to drop and the gas cap spill target dropped to 33 kcfs April 29, resulting in 115 percent TDG in both the tailwater at Lower Granite and the forebay at Little Goose.


The same issues occurred at each lower Snake and Columbia river dam, except at Bonneville Dam where there is no downstream dam forebay (see Snake River overview table at and Columbia River overview table at


Generally, Turner said, water that goes through the powerhouse passes TDG levels that existed in the forebay upstream, but when water is passed over the spillway it creates more TDG.


As the Snake and Columbia rivers become saturated with gas, calculating TDG from the tailwater at one project to the forebay several days travel time to the next forebay can be daunting.


It’s not just spill. TDG reacts with the atmosphere, Turner said, and so there are several other factors that contribute to the levels of TDG in the river, as well: barometric pressure, temperature and wind speed.


As a rule of thumb, as barometric drops, TDG rises by up to 3 percent and it will drop again with a rise in barometric pressure.


Also a rule of thumb, Turner said, is that the percent TDG will rise by 2 percent for every degree the water warms.


And then there is degassing by increasing wind speeds. As wind picks up from zero miles per hour to 10 mph, TDG that is at 115 percent will decline by about 0.2 percent per hour, but as wind picks up even more, its effect on TDG levels rises. A wind change from 0 to 20 mph will drop TDG by 2 percent per hour and wind increases from 0 to 30 mph drops TDG 2.2 percent.


All of these elements contribute to the difficulty of predicting TDG levels in the river and it will continue to change as flows are expected to increase in the coming week to levels that could result in uncontrolled spill (spilling more water than desired to maintain TDG at gas caps because power house capacity has been reached and more water has to be spilled).


For example, as snow begins to melt, total flow at Lower Granite, which on May 2 was about 107 kcfs, is predicted to rise to about 140 kcfs by May 12, a big swing in flow in just one week.


The Dalles flow, which on May 2, was about 327 kcfs is predicted to rise to about 380 kcfs by May 12.


And, each dam is different.


“At McNary Dam, we initially thought that the tailwater would control TDG in the river, but it’s not,” Turner said. “Looking downstream four travel days to the John Day forebay, TDG was actually higher and so the J.D. forebay became the controlling TDG” with spill at The Dalles.


“This is very different for us,” said the Corps’ Julie Ammann. “There is a lot that is out of our control.”


One event this week was expected to drive TDG even higher at Lower Granite Dam. On Wednesday, the day of the TMT meeting, all water passing the dam would have to be briefly spilled as Bonneville Power Administration was making emergency repairs at its substation in the area. That repair called for shutting down all turbines at the dam, prohibiting passing water through the powerhouse.  The Corps expected TDG to rise as high as 131 percent in the dam’s tailrace while repairs were underway.


Storing some water during the repairs in the pool behind Lower Granite, allowing the reservoir to rise above minimum operating pool (MOP), may have tempered the TDG impacts.


Finally, salmon managers at TMT requested a change to the Corps’ spill priority list, moving The Dalles Dam to the top of the list, meaning spill would be initiated at the dam first – before other dams – when the system lacks load.


The spill priority list is used to reduce generation when there is no market for the energy and the only option is to increase spill within the hydrosystem, according to Paul Wagner of NOAA Fisheries.


“This often happens at this time of year when flow is high and loads are low,” he said. “The spill priority list puts a priority on where to put that spill as part of an effort to balance the TDG created by the spill across the hydrosystem.”


The current priority for allocating spill followed the sequence of Lower Granite, Little Goose, Lower Monumental, Ice Harbor, McNary, John Day, The Dalles and Bonneville.


“The request to change that priority to put The Dalles at the top of the list was in response to the desire to have The Dalles spill more than it had been under the current operation when possible,” Wagner said. That’s because The Dalles Dam is not fit with a bypass system for juvenile fish and TMT’s salmon managers believe that when more spill is needed in the system, it should begin at that dam.


BPA has said the additional spring spill to gas cap levels will cost the power system as much as $40 million in lost revenue this year because the water will be spilled and not used for generation. This week BPA Administrator Elliot Mainzer in his quarterly business review said the agency is studying a spill surcharge to recover the cost increases resulting from the spill. He expects BPA to release a plan later this month for public review. He mentioned during the conference call Tuesday, May 1, that cost savings from Fish and Wildlife programs could be an element in recovering those costs.


See more on BPA’s quarterly business review at


Also see:


--CBB, April 13, 2018, “Court Ordered Spring Spill For Fish Begins On Four Lower Columbia River Dams,”


-- CBB, April 6, 2018, “Appeals Court Rules In Favor Of More Spill For Juvenile Salmon, Steelhead At Columbia/Snake Dams”


--CBB, April 6, 2018, “New Court-Ordered Spill Regime Based On Dissolved Gas Caps Begins This Week,”


--CBB, March 23, 2018, “Ninth Circuit Hears Arguments On More Spill For Juvenile Salmon/Steelhead At Columbia/Snake Dams,”


- CBB, December 8, 2017, “Briefs Filed In Appeals Court To Expedite Challenge To Increased Spill For Juvenile Salmon, Steelhead,”

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