EARLY GEOMORPHIC AND FISH-HABITAT RESPONSE TO A UNIQUE LARGE-DAM REMOVAL AND SUBSEQUENT MAJOR FLOODS: CARMEL RIVER, CALIFORNIA

Dam removal is becoming more common as a means to remove liabilities associated with aging dams and to restore anthropogenically altered watersheds. Few large dams (>10 m tall) have been removed, and coupled physical–ecological response studies are rare. We present a 5-year before-after/control-impact study of river response to removal of 32-m-high San Clemente Dam (Carmel River, CA) in 2015, the third-largest intentional dam removal and the first in a Mediterranean hydroclimate. Two factors make this unique among large dam removals: most reservoir sediment was sequestered in place via an engineered channel reroute, with only a minor proportion moving downstream; and high flows (two 10-yr floods and a 30-yr flood) occurred ~1 year after dam removal, increasing annual suspended-sediment load 20-fold compared to the previous year. In the first year post-removal (2015–16), when average runoff ended a multiyear drought, a sediment pulse filled pools with sand in the first 3.5 km downstream of the dam site. Over the second winter after dam removal (2016–17), high flows moved substantial gravel and large wood downstream of the dam site. These geomorphic changes increased the availability of spawning-sized gravel and enhanced channel complexity, which should improve habitat for multiple life stages of steelhead (O. mykiss) and raise population resilience. Densities of juvenile steelhead decreased over the time period ending just prior to the high flows within impacted reaches, whereas mean fish size apparently increased concurrently. We infer that when most reservoir sediment remains impounded, high flows become more important drivers of geomorphic and habitat change than is dam removal alone. High flows soon after dam removal substantially altered fluvial evolution in the upper part of the reservoir, promoting new avulsion rather than the incision and lateral migration that dominated fluvial processes in other former reservoirs. However, despite partial habitat restoration, sediment-supply limitation evident in turbidity records indicates that watershed processes are not entirely restored, presumably because another dam remains upstream. Our findings support the episodic model of river habitat dynamics in Mediterranean hydroclimates.