Washington and Oregon have both adopted revisions to their water quality standards that will allow operators of the eight lower dams on the Snake and Columbia rivers and five mid-Columbia dams to increase spill to create up to 125 percent total dissolved gas (TDG) saturation during the spring spill season.

Washington’s rule change to increase TDG must first be approved by the U.S. Environmental Protection Agency, and if approved would become a permanent option during spring spill from April 3 to June 20. The new rule went into effect Jan. 30.

In Oregon, the Environmental Quality Commission voted Jan. 24 to temporarily modify its water quality standard during spring spill, for five years.

Higher TDG levels can cause a condition known as gas bubble trauma in aquatic life, including salmon.

Both states say the new standards support a flexible spill agreement between state, tribal and federal governments that provides more spill at the four lower Snake and four lower Columbia river dams for 16 hours each day to boost juvenile migration in the spring, while allowing for lower spill and increased generation for 8 hours a day when energy prices are higher. Last spring under the agreement, the eight dams spilled up to 120 percent TDG for 16 hours a day in the eight tailraces. This spring—if Washington’s change is approved by the EPA—that would increase to 125 percent.

In Washington, the change also follows a recommendation of the Southern Resident Orca Task Force, which identified increased spill as a way to boost salmon returns in the Columbia Basin.

"The goal of this rulemaking is to allow more water to spill over dams, which scientific modeling predicts will improve survival of out-migrating juvenile salmon, without undue negative impacts to salmon and other aquatic life in rivers," a news release from Ecology said.

Hydropower operators have the option of using previously established TDG levels—which limit spill to 115 percent in the forebays and 120 percent in the tailrace, with 125 percent maximum level—or using the new 125 percent criterion in the tailrace once Ecology approves a biological monitoring plan, the news release said.

"Hydropower projects that choose to utilize the 125 percent tailrace criterion must follow all applicable Endangered Species Act requirements associated with their spill and fish passage obligations," the release said.

"We have submitted the rule package to EPA for final approval, which includes ESA consultation with the U.S. Fish and Wildlife Service and NOAA Fisheries," the release stated.

Oregon adopted its new TDG levels for five years instead of two—which was recommended by its Department of Environmental Quality—because the longer term allows more time to evaluate whether higher spill is beneficial to salmon and steelhead survival rates, Paula Calvert, DEQ’s Columbia River coordinator, told NW Fishletter.

As recommended, the modified TDG limit will increase from 120 percent to 125 percent as measured in the tailraces from April 1 through June 15 each year.

The new standards also allow maximum TDG levels of 127 percent for two consecutive hours—slightly higher than Washington’s 126 percent TDG two-hour consecutive maximum—to provide the Corps more flexibility in attempting to reach the 125 percent TDG gas levels spelled out in the spill agreement, Calvert said.

Oregon’s action pertains only to the four lower Columbia River dams which are located along a stretch of river that borders Washington and Oregon. The four Snake River dams are in Washington, so modifications to TDG standards there are determined by Washington state water quality rules.

The commission approved modifying the statewide standard for total dissolved gas of 110 percent at the four dams to 125 percent from April 1 through June 15; and to 120 percent from June 16 through Aug. 31. Spill must be reduced when gas bubble trauma exceeds specific thresholds in salmon and steelhead, and in resident fish. “Physical monitoring must occur and be adequate for implementing the requirements of this order,” it says.

Total dissolved gas is regulated in water quality standards because too much gas—mainly nitrogen—trapped as air bubbles in supersaturated water is harmful to fish and other aquatic species.

A memo from the Oregon DEQ recommending the modified standard to its commission notes TDG levels above 110 percent can cause gas bubble trauma in fish. “Based on monitoring from 1995 to present, the incidence of gas bubble trauma in salmon smolts due to spill is approximately 1 percent when total dissolved gas levels are managed to 120 percent below the dams in tailraces,” the memo says.

DEQ’s draft order to approve the modification says that increasing TDG to 125 percent during spring juvenile migration will pass more juvenile Snake River spring/summer Chinook and steelhead over spillways, reducing their passage through turbines and bypass systems.

Juvenile salmon that pass through the powerhouse—either past turbines or via juvenile bypass systems—have lower in-river and early ocean survival, according to the Comparative Survival Study. Monitoring for gas bubble trauma in juvenile salmonids has shown that TDG levels must reach close to 130 percent or higher before the threshold of 15 percent gas bubble trauma in salmonids occurs, the draft order says.

“Based on these considerations, the Commission finds that failure to approve the modification requested by the Corps would, on balance, result in greater harm to salmonid stock survival than would approval of the modification,” the draft order says.

But increasing TDG also raises concerns for resident fish and other aquatic life. “Studies have shown instances when greater than 15 percent of resident fish examined have signs of gas bubble trauma when exposed to 120-125 percent total dissolved gas,” the draft order says. However, field observations indicate that juvenile salmonids are more sensitive than the resident species examined.

Calvert said the commission’s approval of higher total dissolved gas for five years comes with a requirement for the DEQ to report back after two years with results of monitoring for gas bubble trauma for both salmonids and resident fish. The Corps, she said, will conduct the monitoring and share results with both Washington’s Department of Ecology and Oregon’s DEQ.

According to the draft order, the Corps uses real-time total dissolved gas observations at monitoring stations in the tailraces of McNary, John Day, The Dalles and Bonneville dams to determine TDG levels. Juvenile salmonids are collected at Bonneville and McNary dams, examined for gas bubble trauma and ranked based on the severity of their symptoms. “Monitoring non-salmonid species for gas bubble trauma can occur by utilizing the existing infrastructure for sampling juvenile salmonids,” the draft order says.

Before recommending the increase in total dissolved gas to its commission, the DEQ accepted public comments on its draft proposal. It received 14 comment letters and oral testimony with suggestions for changes. Summarizing those comments, DEQ noted two-thirds of commenters suggested a five-year term, while one-third agreed with the two-year recommendation.

Several comments raised questions about plans for monitoring gas bubble trauma in salmon and steelhead, and in resident fish. In response to the comments, DEQ recommended monitoring salmonids and nonsalmonids during voluntary spill above 120 percent TDG, but suggested removing a sample size of 100 fish, and instead implement a weekly minimum sample size of 50 salmonids, and 50 nonsalmonids.

The agency also recommended that annual reporting from the monitoring should include an evaluation of gas bubble trauma in nonsalmonids.

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.