Estimating Fluvial Sediment Mass Flux In The Lower Klamath River Basin Prior To Removal of Four Hydroelectric Dams, California

The transport and deposition of fluvial sediment following dam removals affects river function, including geomorphic processes and alterations to riverine biota, and has many implications for river management. Although many dam removal projects have examined river response to sediment transport following dam removal, few have had the ability to examine pre-dam removal sediment transport to characterize baseline conditions. Four hydroelectric dams are currently scheduled for removal on the Klamath River beginning in winter of 2024, constituting the largest dam removal project in U.S. history. Sediment stored in the impoundments behind the four dams is estimated at approximately 10 million m3 and is primarily comprised of fine-grained material (particle size <= 0.075 mm) with high moisture content. The transport of these reservoir sediments may have far-reaching impacts to the 312-km river corridor downstream of the lowermost dam, including the Klamath River estuary. In anticipation of this large dam removal project, suspended-sediment samples and turbidity data have been collected since 2018 at four mainstem USGS river gages by staff from the USGS, Yurok, and Karuk tribes. Surrogate regression models using turbidity as an independent variable were used to estimate time-series of suspended-sediment concentration (SSC) at sites where adequate data were collected for statistical analysis. SSC were combined with time-series streamflow data to compute mass flux of fluvial sediment. Sediment flux from major tributaries to the Klamath river were estimated from prior studies, and all available sediment flux data were combined to provide a pre-dam removal assessment of sediment transport in the mainstem Klamath River. Comparisons of sediment flux between mainstem gages were conducted to evaluate reaches within the river corridor that may act as sediment storage or transport reaches, which can help identify river reaches that are more prone to experiencing changes to geomorphic processes and associated effects on habitat and biota because of the dam removal efforts.