Klamath Dam Removal Drawdown Scenario 8: Potential Impacts of Suspended Sediments on Focal Fish Species with and without Mechanical Sediment Removal

Sediment transport modeling of the impacts of dam removal on suspended sediments in the lower Klamath River indicates high short-term loads immediately downstream of Iron Gate Dam under the Proposed Action (Greimann et al. 2010, Stillwater Sciences 2008), where the latter is defined as the full removal of all four dams. The potential effects of the Proposed Action on suspended sediment concentrations (SSC) downstream of the dams may be partially mitigated by physical removal of sediment from the reservoirs prior to dam deconstruction. Sediment removal would potentially decrease erosion of the existing reservoir sediment deposits following dam removal and correspondingly decrease the magnitude of suspended fine sediments transported downstream to the lower Klamath River. In support of the EIS/EIR evaluation of dam removal effects on downstream water quality and key fish species, this technical memorandum documents modeling results for a potential sediment removal mitigation measure, including a comparison of model results based on two similar, but not identical, reservoir drawdown scenarios. Details of the Proposed Action and multiple reservoir drawdown scenarios, as well as the approach to mechanical sediment removal from the reservoirs, will be documented elsewhere by the Bureau of Reclamation (USBR) and CDM. Only information relevant to the comparison of the two sediment removal drawdown scenarios for SSC model results and anticipated impacts to focal fish species are summarized below.