Research Sheds Light on Sediment Impacts as Klamath Sees Historic Dam Removal

The Klamath River, currently experiencing the largest dam removal project in U.S. history, is facing significant sediment releases that have coated portions of the riverbed with several inches to feet of fine silt and clay. A comprehensive research review published in Reviews in Fisheries Science provides critical insights into how these sediment levels may impact salmon reproduction.

The meta-analysis by Jensen et al. reveals that salmon egg survival drops precipitously when fine sediments smaller than 0.85 millimeters exceed just 10% of spawning gravel composition. With current sediment deposits on the Klamath far exceeding these studied thresholds, the findings raise important questions about salmon reproduction in affected areas.

The research quantifies specific impacts: for each 1% increase in fine sediments, the odds of survival decrease by 16.9% for Chinook salmon and 18.3% for coho salmon – both key species in the Klamath River system. The study found that coho salmon, an important Klamath species, showed the steepest decline in survival per unit of sediment increase among all species studied.

According to the research, fine sediment affects salmon survival through two primary mechanisms. First, it can impede the flow of oxygenated water through gravel in egg pockets, potentially causing suffocation. Second, layers of fine sediment can reduce interstitial spaces and physically prevent fry from emerging.

The current situation on the Klamath, with several inches to feet of fine silt and clay coating the river bottom in some areas, represents sediment levels far beyond those examined in the studies reviewed. The researchers specifically caution that “extreme caution should be used if one must make estimates for stream types and conditions not represented in the original data.”

The removal of four dams on the Klamath has released millions of cubic yards of accumulated sediment into the river system. This represents one of the largest sediment releases in river restoration history, creating conditions that exceed the parameters studied in most salmon survival research.

The research team, led by scientists from NOAA Fisheries’ Northwest Fisheries Science Center, notes that their findings can help forecast the effects of watershed management practices on salmonids and assist in prioritizing restoration efforts. However, they emphasize that their models are based on studies with lower sediment levels than those currently observed in the Klamath River.

The study’s authors indicate that their research “can be used to forecast effects of watershed management practices on salmonids and to make comparisons between predicted salmonid survival rates under alternative management strategies for conditions where fine sediment is the limiting factor for survival.”

With the Klamath River experiencing unprecedented sediment conditions following dam removal, this research underscores the importance of continued monitoring and potential management interventions to protect salmon spawning habitat. The massive scale of sediment release in the Klamath system presents a unique case study in understanding how extreme sediment events may affect salmon reproduction and survival.

Literature of Interest. (2009). Reviews in Fisheries Science, 17(3), 423. https://doi.org/10.1080/10641260802690010