Affiliation:
1. U.S. Geological Survey Santa Cruz CA USA
2. NOAA Southwest Fisheries Science Center Santa Cruz CA USA
3. Applied Environmental Science California State University, Monterey Bay Seaside CA USA
Abstract
AbstractMeasuring river response to dam removal affords a rare, important opportunity to study fluvial response to sediment pulses on a large field scale. We present a before–after/control–impact study of the Carmel River, California, measuring fluvial geomorphic and grain‐size evolution over 8 years, six of which postdated removal of a 32 m‐high dam (one of the largest dams removed worldwide) and included 11 flow events exceeding the 2‐year flood magnitude. We find that the reservoir‐sediment pulse following dam removal was relatively small (97 000 ± 24 000 t over 4 years), owing to deliberate reservoir‐sediment stabilization. Scaled to the size of the Carmel River watershed and compared against long‐term bedrock denudation rates, the post‐dam‐removal sediment release was slightly less than the annualized long‐term sediment export from this basin. New sediment transited >30 km to the river mouth in less than 2 years, assisted by floods 2 and 4 years after dam removal. The sediment pulse fined the downstream riverbed while causing mostly low‐magnitude bed‐elevation changes: commonly 0.5 to 1 m or smaller, occurring as discontinuous sediment patches or interstitial deposits, aside from the filling and subsequent partial scour of deep pools. There was no major geomorphic reset downstream from the dam site. Geomorphic changes were driven almost entirely by flow rather than by the modest increase in sediment supply, in contrast to recent examples from other large dam removals. The relatively minor disturbance caused by dam removal on the Carmel River is likely analogous to many future dam removals: a relatively small sediment pulse after deliberate limitation of reservoir‐sediment erosion, and with an upstream dam remaining in place. Thus, a large dam removal need not lead to major downstream impacts.
Funder
National Oceanic and Atmospheric Administration
U.S. Geological Survey
California State University, Monterey Bay
Subject
Earth and Planetary Sciences (miscellaneous),Earth-Surface Processes,Geography, Planning and Development
Cited by
3 articles.
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1. A Watershed Moment for Western U.S. Dams;Water Resources Research;2023-10
2. Coastal vegetation responses to large dam removal on the Elwha River;Frontiers in Ecology and Evolution;2023-09-29
3. Rivers;Landscapes of the Anthropocene with Google Earth;2023