Fine sediment in a coarse-bedded river: geomorphic response of the Klamath River to dam removal

Year: 2022
Presenter/s: Chauncey Anderson
Symposium Session: 2022 - 06 Klamath River Dam Removal is the Largest and Most Complex Fish Restoration Project Ever Attempted
Topics covered: fish passage, fish-salmon, hydraulics, instream structure (culvert/bridge/dam), modeling, risk, and sediment transport


ABSTRACT

The USGS is engaged in a multi-phase study to collect baseline data and to investigate the effects of dam removal on sediment transport, habitat and the geomorphologic features of the mainstem Klamath River. Four hydroelectric dams are slated to be removed in 2023 to improve flow, water quality, sediment transport, volitional fish passage and salmonid habitat for endangered and threatened salmonid species in the basin. The four dams currently store approximately 10-12×106 m3 of mostly fine sediment (~85% clay or silt). The simultaneous removal of these dams will deliver a substantial fraction of the stored sediment to over 190 miles of unregulated river downstream. Most of the released fine sediment is expected to be transported in suspension to the river mouth within two winters. In contrast, the coarse sediment is predicted to move more slowly, as suspended and bedload depending on flow conditions, resulting in more dynamic geomorphic processes in reaches where habitat is currently degraded by the presence of the dams. The USGS is developing a sediment budget prior to dam removal to provide a baseline for comparison to a post removal sediment budget, and to quantify the amounts and characteristics of sediment transported from the system or deposited along its length. In addition to yielding results that will be used to assess the effects of dam removal, the study is investigating new methods to track pulses of fine-sediment (sand/silt/clay) through coarse-grained river corridors (cobble/boulder) including the differentiation of sediment sources (e.g. reservoir sediments versus downstream tributary sources).
Several complementary methods are being used to characterize and quantify the baseline sediment budget. A cornerstone of this effort is a2 018 integrated topo-bathymetric terrain dataset, which we are using to select intensive study reaches distributed downstream of the lowermost dam along the mainstem river corridor. The system-wide terrain dataset provides context to the data collected at the intensive study reaches. We are using traditional methods (e.g. gaged data, bed-material sampling, and vegetation, habitat and geomorphic mapping) and remote-sensing technologies (e.g. sedcams, sonar, UAV surveys, and LiDAR) to monitor fine-sediment dynamics (e.g. bed-fining and changes in sediment storage) along the river corridor. We are using sediment tracing methods (fingerprinting, macroinvertebrate bioaccumulation of metals) to detect and differentiate reservoir sediment from tributary, landslide, and other sources downstream of the dams. The combination of approaches provides an assessment of pre-dam-removal sediment loads and geomorphic attributes of the river, setting the stage for evaluation of the effects of dam removal on these attributes. This presentation will include a discussion of the methods and monitoring strategies to predict and document sediment dynamics at the intensive study sites and will discuss implications of dam removal for restoration of riverine processes.