Sediment transport assessment and dynamics during and after the largest dam removal in U.S. history on the Klamath River, Oregon and California, USA

In many regions worldwide, dam removal is being considered as a means to restore rivers and to remove hazards and liabilities associated with aging infrastructure. The pace and scale of dam removals has increased exponentially in the past two decades, providing a rapidly growing knowledge base with which to evaluate the consequences and effectiveness of breaching and removing dams. The largest dam removal in U.S. history on the mainstem Klamath River in Oregon and California, USA, has presented novel suspended-sediment transport conditions by giving the river access to sediment accumulating in the reservoirs since 1918.  Three large dams were removed simultaneously in 2024, and one low-head dam was removed in 2023. Turbidity monitoring and suspended-sediment concentration (SSC) sampling were conducted before, during, and after the dam removals as part of an inter-agency collaborative effort that included the dam removal entity (Klamath River Renewal Corporation), private consultants, the Karuk and Yurok indigenous tribes in California, and the U.S. Geological Survey (USGS).  These data were used to generate ordinary-least-squares regression models to compute time series of SSC and suspended-sediment loads at six mainstem USGS streamgages spanning 300 river kilometers downstream of the former dam sites.  The reservoir drawdowns prior to dam removal introduced large amounts of fine-grained sediment into the coarse-grained river corridor causing elevated turbidity and peak SSC of approximately 30,000 mg/L.  Multiple stages of the dam removal process, including reservoir drawdown, geomorphic flows for sediment mobilization, and the breach of historic cofferdams, resulted in dynamic sediment-transport conditions.  This work provides insight into differences between fine-sediment transport related to dam removal and natural sediment transport events in this large 40,000 km² basin.