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The students, enrolled in the course “Restoration of Rivers and Streams,” spent the day learning about the response of the Carmel River to the removal of the San Clemente Dam two years ago. Demolition of the 32-meter (105-foot)-high dam, completed in November 2015, was the third-largest dam removal in the U.S. so far, and the first removal of a large dam in California. East and Williams showed the students the former dam site and evidence of change and recovery both up- and downstream.

The concrete arch dam, built in the 1920s to supply water to the Monterey Peninsula, was removed because it had been declared unsafe and because its reservoir had filled with sediment and no longer provided water storage. This is one of a growing number of instances in the U.S. where obsolete or unsafe dams are being removed to mitigate economic and safety liabilities, as well as to restore river habitat. In the Carmel River, removal of San Clemente Dam is expected to improve habitat conditions for two species listed as threatened under the Endangered Species Act: steelhead fish (anadromous Oncorhynchus mykiss) and red-legged frogs (Rana draytonii).

Scientists from the USGS, NOAA, and California State University Monterey Bay have been studying the Carmel River before, during, and after dam removal to measure the physical and biological response. The teams measure channel topography, turbidity in the water, and sediment particle size on the riverbed, and they document the numbers and sizes of fish using various parts of the river.

Most of the sediment trapped in the former reservoir was intentionally left in place after dam removal to prevent increased flood hazard downstream, which might have resulted if a large quantity of sediment were to accumulate on the floodplain. Some sediment did move downstream, however, having eroded from the part of the reservoir above the sequestered material. Although the effects on fish and frog populations may take a decade or more to document fully, the ecosystem response thus far appears to be positive. As Williams and East explained to the students, the new sediment, combined with major floods during the winter of 2017, filled pools in the riverbed downstream of the dam site and deposited new gravel that now provides good habitat in which steelhead can spawn. The scientists have measured an increase in suitable spawning habitat in 2017 and have observed steelhead over a broad range of life stages (ages) using the newly reworked river channel, both above and below the dam site.

At the end of the field trip, one student commented that the opportunity to see the former San Clemente Dam site and the recovering Carmel River was “what I really wanted to get out of this class.”