Removing dams can restore natural river flows, improve water quality and ecosystem function, and redistribute accumulated sediments to downstream environments. However, the volume of sediment released during large dam removals can be enormous, causing relatively short-term but significant impacts to the fluvial system and surrounding ecosystems. 

The carefully planned takedown of the San Clemente dam on the Carmel River in November 2015 offers an alternative approach to large-scale dam removal that minimizes impacts downstream.

After impounding water and sediment in the San Clemente reservoir for almost a century, the San Clemente dam was at the end of its useful life. The reservoir was filled with 1.7 million cubic meters of sediment, which left little room for water storage. There was also the potential for dam failure during an earthquake or large flood.

The dam removal was carried out with the intent to minimize downstream impacts by limiting reservoir sediment erosion. Reservoir sediment kept in place wouldn’t aggrade, or build up, on the riverbed downstream, which would increase flood risk to structures built on the Carmel River floodplain. As planned, the upper third of the 1.7 million cubic meters of reservoir sediment was accessible for natural fluvial erosion after dam removal, whereas the lower two-thirds were sequestered as the river was rerouted into a tributary channel, San Clemente Creek, to bypass the lower part of the former reservoir. 

By tracking geomorphic changes to the riverbed following the San Clemente’s removal, researchers found that the reservoir-sediment pulse was indeed relatively small, causing no major changes to river morphology downstream from the dam site. The response of the Carmel River to dam removal was much more muted than on the Elwha River, the only other large dam-removal project with long-term monitoring data for direct comparison.

“The success of the San Clemente dam removal may open up new possibilities for the removal of large dams without causing significant downstream impacts,” said Amy East, USGS Research Geologist and lead author of the study. “This approach could be replicated in other parts of the world where dam removals are necessary, but could pose significant impacts to downstream environments if not managed carefully.” 

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