Would tearing down the four lower Snake River dams help spring Chinook salmon and steelhead? The new assessment from NOAA Fisheries indicates significant uncertainty as to how much it would help.

Released last month, the “ESA Recovery Plan for Snake River Spring/Summer Chinook Salmon & Snake River Steelhead,” assesses the threats to Chinook and steelhead in the Snake River Basin. The analysis examines the major factors affecting the runs, including the impact on the dams and their ability to meet the “performance standards of 96 percent survival for yearling Chinook and steelhead migrants.”

Although the report indicates threats from the dams, there is a great deal of good news for Chinook and steelhead.

First, NOAA notes that while the dams continue to impact Chinook and steelhead, the impacts “have been significantly reduced in recent years, especially for steelhead.” Several improvements have improved the ability of Chinook and steelhead to pass the dams. NOAA Fisheries writes the dams, “are very close to achieving, or have already achieved, the juvenile dam passage survival objective of 96 percent for yearling Chinook salmon and steelhead migrants…”

Second, the report is equivocal about how destroying the dams would impact the fish. For example, removing the dams would reduce the travel time for juveniles downstream, increasing their chance of survival.

Chinook and steelhead, however, could also be harmed by dam removal.

The NOAA authors write, “Turbidity would increase dramatically for the first several years with much of the sediment transport occurring in the spring months. Juveniles migrating in the spring would experience highly turbid conditions.” The impact on temperature would be complex, decreasing the minimum temperature in winter, but “increasing maximum summer temperatures.” How this would play out is speculative, with NOAA noting it is unclear whether temperatures would be cooler or warming as juveniles travel downstream when they are most vulnerable.

Further, juvenile fish could actually see increased predation. The report authors note, “Migrating smolts would be less exposed due to decreased travel times through the lower Snake River, but, at least initially, the large existing population of predators would be concentrated into smaller volume of the unimpounded river, potentially increasing predation rates.”

It may be that destroying the dams would increase fish populations more rapidly than is already occurring. How much, however, is uncertain and some threats to juveniles may increase, including increased turbidity, predation, and even temperature.

Policymakers must assess the tradeoff between the uncertain gains to Chinook and steelhead and the high cost of lost energy, flood control, transportation, and other benefits. We are certain of the very high cost of removing the dams. What the NOAA analysis shows, is that the benefits of tearing them down are far less certain. This is especially true since some of the dams are already seeing survival rates of 96 percent. Even in the best-case scenario where juveniles have a nearly 100 percent survival, the marginal gain seems very small.

Unless the science improves and is more certain Chinook and steelhead would benefit, tearing down the dams is a very high cost risk for extremely uncertain benefit.