Klamath Reservoir Sediment Characterization and Drawdown Impacts for the Dam Removal Investigation

A multi-agency study is currently underway to determine whether the removal of four dams on the Klamath River will advance restoration of the salmonid fisheries of the Klamath Basin and is in the public interest. To assist in this decision, multi-disciplinary scientific assistance is being provided to review existing information and complete additional studies where needed. A portion of the scientific team is determining the predicted sediment impacts for removal of the dams and implications to vegetative restoration. The sediment in the reservoirs was characterized based on soil properties, grain size, desiccation properties, and critical shear stress. In addition, reservoir surveys and geotechnical borings were used to determine the sediment thickness, volume and spatial variability in each reservoir. A relationship was found between sediment depth (from geotechnical drill holes) and position within each reservoir to calculate the thickness. The soil properties, including grain size and critical shear stress, were determined from laboratory testing. Historical aerial photography was also used to ascertain the historical changes to the river and potential future changes. Once the sediment distribution and characteristics were decided for each reservoir, a number of numerical model simulations were completed to determine the impacts of different drawdown runs. A one dimensional (1D) numerical hydraulic and sediment transport model (Sedimentation and River Hydraulics - One Dimensional model, SRH-1D) was developed of the three reservoirs and river segments in between. The bathymetry data collected in the reservoirs in 2002 was used with the sediment thickness determination to develop the geometry in the reservoirs. LiDAR data collected in 2010 was utilized for the channel geometry between the reservoirs. Three hydrology simulations were developed for the deconstruction period. All simulations start on November 15th , 2019 and are 2-year time series of daily flows. Historical flows from 1961-2009 were used to find hydrographs for a dry (10% non-exceedance), median (50% non-exceedance), and wet (90% non-exceedance) water year, which were followed by the next water year hydrograph to develop the time series. The dry year had high sediment concentration (> 1,000 mg/L) from the start until January. The concentrations dropped temporarily but spiked again during the end of the spring runoff (in June). There is also a potential for higher concentrations in the second year following deconstruction because less sediment was eroded during the first. The median and wet years had high concentrations from the start until December as well as intermittently high concentrations through May and again in June and July after the end of spring runoff. However more sediment eroded in the wet year than the median year. The desiccation properties were applied to the sediment thickness maps to create maps of the post-dam reservoir topography. These maps were used in combination with vegetation surveys, vegetation studies, and historical information to predict areas suitable for riparian, wetland, and upland vegetation restoration after dam removal.