An intriguing question has come up in the power struggle between hydropower and wind energy in the Northwest:

Has Bonneville Power Administration really hit the limit of how much water it can spill over Columbia Basin dams without gassing salmon to death – or is that just an excuse to shut down wind turbines and generate more hydropower?

With few options for managing a deluge of spring runoff this year, BPA says it needs to reserve the right to turn off wind turbines and clear the grid for hydropower to avoid overgeneration (which can be costly) and to protect salmon from gases generated by too much spill water.

BPA can scale back hydropower generation (and make room for more wind power) by spilling extra water over the dams, but only up to the point where the spill creates so many gas bubbles in the water that it hurts fish. How many gas bubbles is too many? And has BPA reached that limit?

Fish biologist Margaret Filardo, who monitors the dams from the Fish Passage Center, says legally the answer is yes. Spill in the basin now is exceeding the 120 percent dissolved gas limits set by Oregon and Washington under the Clean Water Act.

But those limits are violated all the time, she says, and scientifically the agency could spill more without raising dissolved gas levels to the point where they would harm salmon. She only starts to worry when the levels hit 130 percent.

The science of gassing salmon

At the 130 percent dissolved gas level, significant numbers of fish can start getting the bends, Filardo said. The fish take the supersaturated water in over their gills, and it elevates the level of dissolved gasses in their bloodstream.

Like a diver, the fish are OK when they’re deep in the water column because the pressure decreases the dissolved gas level. But as the fish comes up to the surface and the pressure is removed, the gas bubbles out into their fins and their gills. When the bubbles cover 25 percent of the fish or more, scientists have found it can hurt fish survival.

“Where this system gets into trouble is when you have extremely high flows and very high levels of total dissolved gas,” Filardo said. “We start observing fish with lots of bubbles. Research has shown as you get more bubbles, the chances are you’ll have higher mortality. “

At 125 percent dissolved gas, she said, the fish are usually moving through the system so fast they don’t have time to absorb enough gas to be harmed.

Gas limits = spill limits

The dissolved gas limits BPA is using now were set in the early 1990s, when salmon runs were very low and federal fisheries scientists were looking at spilling water over dams to help fish passage. Controlled spill hadn’t been used deliberately to help fish before, Filardo said. And dam operators needed permission to create higher levels of dissolved gas than the 110 percent water quality standard.

Federal and state agencies agreed to the 120 percent limit. But since then, scientists have collected a lot of data on how much dissolved gas fish passing over the dams can actually handle.

“It looks like probably 125 percent would’ve been the right number to go with at the time,” Filardo said, “but 120 is the standard. What that does is it limits the amount of spill.”

It also puts a cap on the economic losses of spilling water over dams as opposed to putting water through turbines, she said.

“Technically is BPA legally doing what’s right? Yeah,” said Filardo. “Is it because of fish? Well…”

Raise the limits?

BPA could request that the states raise the limit for dissolved gas, she said. That would allow the agency to spill more and allow more wind power onto the grid during the spring freshet.

But BPA spokesman Micheal Milstein said BPA is not a water quality agency and is following court orders in observing the current dissolved gas limits.

“It would be extraordinarily unusual for federal agencies to get involved in a discussion of  a state standard like that,” he said. “There’s definitely some benefit to fish from spill up to a point, and when you get up in those higher gas levels – particularly for extended periods of time – wheres the trade-off? When does harm start to exceed the benefit? We don’t feel that’s a place we should be engaging.”

A secondary dissolved gas limit for water downstream from the dams is different in Oregon and Washington and is currently being disputed in court. Environmentalists are challenging Washington’s lower standard of 115 percent, which sets a lower limit for spill for the entire basin in spite of the fact that Oregon’s standard is 120.

Filardo said the Fish Passage Center argued that the 115 percent downstream standard was unscientific, while the U.S. Army Corps of Engineers and BPA presented “contrary, or adversarial information.” And Oregon and Washington came to different conclusions on who was right.

But Milstein said the extra spill generated by a change in that downstream standard wouldn’t be enough to resolve the wind vs. water conflict in a high spring water year like this one.

“Both we and the Fish Passage Center did analysis of what would happen if we ignored the more restrictive Washington standard,” he said. “How much difference would that make in volume spilled? We both concluded the difference wouldn’t be that great.”