Geotechnical failure events and channel morphology in Copco I and Iron Gate reservoirs during the Klamath Dam removals

Draining reservoirs, a process known as drawdown, is increasingly implemented to maintain or decommission dams, to promote fish passage, and enhance water quality, among other objectives. Understanding the dynamics of sediment erosion during reservoir drawdown is critical for achieving those varied management objectives. This study analyzed over 35,000 timelapse images of channel evolution and sediment erosion at two reservoirs on the Klamath River to examine the factors governing channel evolution for the first five months following drawdown. Large volumes of fine sediments adjacent to the channel moved laterally into the emerging channels during drawdown while the soils were wet, accompanied by vertical erosion of the river channel to the historical bed. The dramatic lateral movements of sediment, similar to the processes that drive landslides and henceforth undocumented in draining reservoirs, concluded after the reservoirs were drained. Flow releases from upstream intended to promote additional erosion mobilized sediment stored in the channel but did not trigger appreciable lateral movement of sediments. Archived data include the geotechnical failure event logs used in this analysis for Copco I and Iron Gate reservoirs, and the channel widths and water surface elevations derived from a terrestrial laser scan at Iron Gate reservoir.