Early Geomorphic and Fish-Habitat Response to a Unique Large-Dam Removal and Subsequent Major Floods: Carmel River, California

East, Amy E.; Harrison, Lee R.; Smith, Douglas P.; Bond, Rosealea M.; Logan, Joshua B.; Nicol, Colin; Williams, Thomas H.; Boughton, David A.

doi:10.1130/abs/2017am-296267

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“…The Carmel River showed changes in riverbed morphology and grain size throughout the 6 years studied after dam removal (Figures 4-9; data available at https://doi.org/10.5066/P9HG8UDS; East et al, 2022). All nine impacted reaches consistently showed largermagnitude changes than did the control reach (CR; Figures 5 and 7A).…”

Section: Resultsmentioning

confidence: 98%

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Six years of fluvial response to a large dam removal on the Carmel River, California, USA

East

Harrison

Smith

et al. 2023

Earth Surf Processes Landf

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Measuring 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.

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Section: Resultsmentioning

confidence: 98%

“…Panel labels indicate reach name and transect number (e.g. CR2 is transect 2 of the control reach), using the same naming convention as in the associated data repository(East et al, 2022). Transects were selected to show some of the more substantial geomorphic changes in each reach.…”

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confidence: 99%