Washington's Department of Ecology issued a short-term modification of the state's total dissolved gas (TDG) standards at eight dams on the Snake and Columbia rivers on March 29, enabling the U.S. Army Corps of Engineers to begin flexible spill operations April 3.

The decision increases TDG limits from 115 percent to 120 percent at forebays and maintains a 120 percent TDG at tailraces of four lower Snake River and four lower Columbia River dams.

The limits had been restricted to 115 percent in forebays of these dams--a standard that remains in the lower Snake and Columbia rivers when spring spill is not in effect. High levels of supersaturated gas can injure or kill fish and other aquatic life, and is limited to 110 percent in most other rivers of the state.

An April 2 news release from the Corps says the flexible spill operations will help the region learn whether higher spill improves juvenile salmon and steelhead survival, and whether it improves adult returns in years to come.

"We will use our expertise and best professional judgment to implement this operation," Tim Dykstra, senior fish program manager for the Corps, said in the release. "And since this operation involves spilling much more water at our dams for juvenile fish passage than in previous years, we will monitor the river system closely and adjust as necessary if we see any unintended consequences from the higher spill."

The agency expects to post much of the technical information at its Columbia River Technical Management Team website, with discussions likely during weekly TMT meetings.

The modified criteria increasing TDG to 120 percent applies only to Lower Granite, Little Goose, Lower Monumental and Ice Harbor dams from April 3 through June 20; and McNary, John Day, The Dalles and Bonneville dams from April 10 through June 15. It is in effect in 2019, 2020 and 2021 unless a change in the state's administrative rules supersedes the decision.

A final environmental impact statement (EIS) issued with the decision says that a separate process will begin this summer to address potential rule changes to accommodate a 125 percent TDG limit outlined in the flexible spill agreement for the spring of 2020 and 2021.

According to the EIS, the objectives of issuing the short-term modification is to meet the goals of the flexible spill agreement, to potentially increase smolt-to-adult returns for salmon and steelhead, and to develop a consistent standard with the state of Oregon.

In its EIS, Ecology acknowledges that little is known about actual TDG exposures of aquatic organisms while navigating through river systems; that both laboratory and field studies are limited; and many uncertainties remain.

When considering the potential positive impacts from increased spill, the EIS points to two models. One is the Comparative Survival Study (CSS), a joint project of the Fish Passage Center, the Columbia River Inter-Tribal Fish Commission, the U.S. Fish and Wildlife Service, and wildlife departments in Oregon, Washington and Idaho. Updated annually, the model predicts steady improvements in juvenile survival and adult returns as spill increases to at least 125 percent TDG.

The second study is NOAA Fisheries' Comprehensive Passage (COMPASS) model, which is "less optimistic" about the benefits of additional spill, the EIS notes. The CSS model predicts better juvenile survival "largely because of the assumption of latent or delayed mortality due to powerhouse (i.e., non-spillway) passage routes and different conclusions about the relative benefit of fish transportation as an alternative to spill," it says.

The Independent Scientific Advisory Board reviewed both models, but did not directly compare them, the EIS said. "ISAB seems to find value in both the CSS and COMPASS models, and has generally acknowledged that proponents of each model and of different spill tests have merit," it said.

In terms of potential negative impacts from the increased TDG levels, the EIS says most spawning is unlikely to be affected since there are only a few suitable spawning areas in the main-stem rivers, where TDG levels will be increased.

It reviewed studies examining impacts to juvenile and adult salmonids, and to resident fish, and concluded that several uncertainties remain. Several studies demonstrated that water depth helps compensate for the harmful impacts from high TDG levels, but it's unknown whether fish can detect supersaturated water and purposely seek out deeper waters. In addition, several studies suggest that gas bubble trauma--the main impact observed in monitoring efforts--may not accurately depict the health problems of aquatic life exposed to high TDG levels.

The EIS notes that the entire run of spring Chinook adult salmon and much of the sockeye run are likely to experience the daily elevated levels of TDG. In addition, the vast majority of juvenile salmon and steelhead migration corresponds with the spring spill.

It concludes that increasing TDG levels to 120 percent may increase the duration of exposure that fish have to higher TDG levels, but will not necessarily change the maximum allowable TDG level.

"Given that dam and salmon managers have not previously provided voluntary (fish passage) spill to 120 percent due to the potential for higher TDG levels to increase symptoms of gas bubble trauma in juvenile salmon, steelhead, and non-listed aquatic species; monitoring for gas bubble trauma will continue to be required," it said.

The smolt monitoring program has collected data on juvenile fish conditions and gas bubble trauma in the Columbia Basin since 1995, the EIS says. When more than 15 percent of the juvenile fish show any signs of gas bubble trauma, or more than five percent show severe signs, spill is reduced. "Spill may be curtailed, if possible, when one or both of these action criteria are met," it says.

While approving increased spill to 120 percent TDG, the decision notes that continuing to increase spill may eventually lead to diminishing benefits.

Studies demonstrate that the effects of TDG and the incidence of gas bubble trauma in aquatic life are greater at 125 percent compared with 120 percent, the EIS said. Spilling to 125 percent TDG relies heavily on the ability of aquatic organisms to depth compensate to minimize TDG effects.

When evaluating risk to aquatic life at 125 percent TDG, further research that addresses the uncertainties of the science will help to determine if the potential benefits of spill at 125 percent TDG outweigh the adverse effects of TDG to salmonids and resident aquatic life."

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.