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TMT Monitors Adult Salmon Passage Delay Under New Protocols by K.C. Mehaffey NW Fishletter, May 27, 2026

The decision may be short-lived

A cohort of Snake River spring-summer Chinook may be delayed in the tailrace below Lower Monumental Dam, and the Columbia River Technical Management Team (TMT) agreed during a special meeting May 21 to monitor its progress rather than reduce spill to encourage the fish to pass the dam.

But the decision may be short-lived.

On May 25, federal defendants and plaintiffs in the Columbia River System Operations lawsuit filed a joint stipulation amending U.S. District Judge Michael Simon's preliminary injunction, specifically with respect to adult fish passage delays. The stipulation "shall apply with full force to spring fish passage spill operations at Lower Monumental dam, including as applied to whether to modify Court-ordered spill levels to address adult passage delays." The parties said no court order is required, since terms of the injunction apply until the parties agree to amend or terminate it.

The stipulation says that federal defendants plan to submit a system operational request to the TMT for deliberation.

At its May 21 meeting, TMT discussed whether to change the spring spill operation at the dam, and how to interpret new protocols in the 2026 Fish Operations Plan (FOP) for responding to adult delays at some of the Columbia River System dams.

The 2025 FOP called for decreasing daytime spill to 40 percent of the project outflow for eight hours a day when adult delay or passage issues were observed.

Under this year's FOP, salmon managers and dam operators are directed to "monitor and manage adult conversion rates in-season via [the] TMT process."

In that process, the agencies are asked to use "all available information" to monitor the delay, with no prescribed remedies.

At the TMT meeting, salmon managers suggested that the delay at Lower Monumental Dam is not to a point where a change in operations is necessary, or beneficial.

"Fish are still passing, but maybe not quite as fast as we would expect," said Jay Hesse, who represents the Nez Perce Tribe on TMT. He noted that between 500 and 2,000 fish were still passing the dam daily, and that the delay is shown in data that relies on a handful of salmon with passive integrated transponder tags.

He also noted that flows in the Snake River are likely to drop somewhat over the next few weeks due to scheduled reductions in both Dworshak Dam and Upper Snake River flow augmentations, which could encourage more passage.

No heat waves are predicted and water temperatures were not at levels that would stress fish, he added.

Gabe Scheer, a researcher at the Fish Passage Center, said that some cohorts of fish tend to be slower than others and the apparent delay at Lower Monumental Dam could be due to a slow cohort of fish moving through the system.

In a follow-up memo, Scheer wrote that "prior travel time, temperature exposure, migration timing, and individual condition can all influence subsequent travel time and apparent survival."

He pointed to studies showing "that adult Snake River spring/summer Chinook express different migration behaviors, with some fish moving rapidly through reaches and others moving more slowly."

The memo says that prolonged travel time is mostly a concern when water temperatures are above 62 F, and especially concerning when temperatures exceed 68 F.

"In other words, the concern is not simply that a fish takes a few additional days to move through a reach, but whether that additional time contributes to accumulated thermal exposure associated with reduced survival outcomes," it says.