River response to large‐dam removal in a Mediterranean hydroclimatic setting: Carmel River, California, USA

Dam removal provides a valuable opportunity to measure the fluvial response to changes in both sediment supply and the processes that shape channel morphology. We present the first study of river response to the removal of a large (32-m-high) dam in a Mediterranean hydroclimatic setting, on the Carmel River, coastal California, USA. This before-after/control-impact study measured changes in channel topography, grain size, and salmonid spawning habitat throughout dam removal and subsequent major floods. During dam removal, the river course was re-routed in order to leave most of the impounded sediment sequestered in the former reservoir and thus prevent major channel and floodplain aggradation downstream. However, a substantial sediment pulse occurred in response to base-level fall, knickpoint migration, and channel avulsion through sediment in the former reservoir above the newly re-routed channel. The sediment pulse advanced ~3.5 km in the first wet season after dam removal, resulting in decreased riverbed grain size downstream of the dam site. In the second wet season after dam removal, high flows (including a 30-year flood and two 10-year floods) transported sediment > 30 km downstream, filling pools and reducing cross-channel relief. Deposition of gravel in the second wet season after dam removal enhanced salmonid spawning habitat downstream of the dam site. We infer that in dam removals where most reservoir sediment remains impounded and where high flows follow soon after dam removal, flow sequencing becomes a more important driver of geomorphic and fish-habitat change than the dam removal alone.