OPINION: Misused water spill at dams is the enemy of fish survival

John McKern was a fish and wildlife biologist for the Army Corps of Engineers from 1971 through 2000. He administered many of the research contracts or followed the studies funded by the corps. He worked extensively on modeling fish passage and survival rates with computer models. Much of his career with the corps was spent on projects to improve fish passage and survival at the four lower Snake River dams. For the past 24 years, he has been a consultant on fish passage issues as Fish Passage Solutions, LLC.

McKern also is an adviser to the Citizens for the Preservation of Fish and Dams, Inc. (cfpfd.org). He has compiled the following information for this column.

Adult survival rates of more than 99% are from radio tracking studies by University of Idaho researchers in the early 2000s when they radio-tracked adult spring/summer chinook and steelhead from Ice Harbor Dam to their spawning areas in Idaho. They found 99.2% of spring/summer chinook and steelhead survived.

This parses out to more than 99.7% survival per project (dam and reservoir). McKern’s own observations from many days on the reservoirs over the decades support these findings.

The juvenile fish survival rates at the dams are from the National Oceanic and Atmospheric Administration’s Fisheries and Pacific Northwest Laboratories studies in the late 2010s, where cumulative survival by all passage routes was compiled to provide a survival rate of more than 96% (the NOAA-Endangered Species Act standard) for each of the eight dams the corps operates.

If there were no reservoir mortality, migration through the eight dams (96% to the eighth power), the system survival would be 72%. Those percentages equate to peer-reviewed project survival rates of 89.2% to 84% on average. It seems reasonable that the lower figures, 45% to 65%, are mostly because of reservoir mortality.

Juvenile salmon face mortality from the hatchery or redd to the ocean and back. From hatcheries to Lower Granite Reservoir, mortalities in the free-flowing river range from 40% to 80%, depending on the distance traveled. Fifty percent mortality through the eight reservoirs and dams may be little different than if the dams weren’t there.

From 1965 to 1981, the National Marine Fisheries Service researched transporting juvenile salmon around the dams and reservoirs to:

Avoid mortality in passing the dams.

Avoid predators in the reservoirs.

Avoid migration delay in slower moving reservoirs.

Avoid deadly gas supersaturation.

NMFS and the corps developed the juvenile fish transportation program that moves smolts in barges from Lower Granite, Little Goose and Lower Monumental dams to below Bonneville Dam. For more than 40 years, transportation provided more than 98% survival during the 36-hour trip, compared to the research-documented 50% in-river mortality.

Harvest management agencies led by the Fish Passage Center and ordered by a federal judge have required the corps to decrease hydropower generation and increase the amount of spill to 125% total dissolved gas to provide “safe passage” over spillways. In the 1970s, when gas supersaturation threatened extinction of the Snake/Columbia river salmon, 110% TDG was established as the Environmental Protection Agency and state standard. Now, they say it is “safe” at 125%. And from Lower Granite Dam to the estuary, more than 400 miles, the river is supersaturated up to 125% TDG.

They say that so long as the fish can dive to equilibrate the gas in their blood, it is OK. The research on TDG effects does not support their reckless control of spill.

Many juvenile salmon migrate near the surface where high TDG is more dangerous. In the 1970s, McKern saw all kinds of fish killed by TDG, including juvenile salmon. Adult Snake River salmon coming back to spawn now face supersaturated TDG from well below Bonneville Dam until they pass over Lower Granite Dam. Though many migrate in the upper levels of the reservoirs, they must rise and pass fish ladders at each dam, where the depth is typically six feet. Those that survive the high TDG often show injuries because of its narcotic effects.

In 1999, McKern was on the corps team that determined that if the harvest management agencies were going to require spill, safe spill for juvenile fish should be provided. That gave rise to the raised spillway weir. The intent was to provide one or two spillway bays (depending on dam size) where surface water went over the weir instead of under a gate 40 feet to 50 feet below the surface. Normal spill, pressurized by the depth, entrains air, resulting in elevated TDG, while overflow spill does not. Juvenile fish key in on the overflow spill like they would find in the river flowing out of a lake.

As a result, the overflow spill bays pass five to seven times as many fish, and they speed up the passage of juvenile fish that delay in diving down into normal spillways or into turbine intakes to pass the dams. Instead, the Fish Passage Center and the federal judge require mass spill at all spill bays — not just the raised ones.

The 50% mortality attributed to the lower Snake River and lower Columbia River dams is more a factor of how the Fish Passage Center and the judge are misusing spill.

In the 1990s, it was normal to transport 10 million to 20 million Snake River smolts below Bonneville Dam with 98% survival. In 2024, transport of lower Snake River salmon was reduced to less than 1 million fish as most of the juvenile outmigration was spilled down the river at 125% TDG.

Not only are smolt numbers compromised, but whenever adult survival surges up in ocean returns, the harvest management agencies increase harvest. While Mother Nature, the harvest management agencies and the corps work to get the smolts to the ocean, the harvest management agencies, not the dams, determine how many adults are allowed to spawn.

More than 755,000 sockeye returning in 2024 across nine dams and reservoirs shows that with good spawning, rearing habitat and ocean conditions, salmon runs can return.

The four lower Snake River dams have some of the best fish passage in the world, whereas no dams in Idaho have any fish passage.

Dugger retired as a journeyman carpenter from Clearwater Paper. He lives in Lewiston.