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Speed Bump Slows Tidal Flow, Accelerates Marsh Restoration

Often, when we restore marshes, we take out barriers like culverts or dams because they don’t allow enough water to flow into the marsh. But that wasn’t the case with the Elkhorn Slough project in California: there was plenty of water, and it was flowing in too fast. So we built a barrier—almost like a speed bump—to slow down the flow of water going in and out of the marsh.

On the West Coast, a slough (pronounced “slew”) is an inlet or marsh area with meandering channels and slowly flowing water. Elkhorn Slough, in the Monterey Bay area, was once home to the largest tract of tidal salt marsh in California outside of the San Francisco Bay.
In the last few decades, development—including construction in an adjacent harbor, river diversion, and draining of the marshes—has reduced the amount of marsh by more than half. Water now flows so quickly into the area that it scours away sediment, eroding the banks. Hundreds of acres of salt marsh, now flooded too much of the time, are dying.

That’s where the speed bump comes in. Using heavy machinery, we drove steel pilings into the firm ground beneath the marsh and built an underwater wall, called a sill. The sill slows the flow of water to a more natural rate, while allowing marine mammals and fish to easily pass over at all tides. This sill will help to prevent further erosion and allow native marsh grasses to grow again.

This project, which received $4.5 million in Recovery Act funding from NOAA in 2009, enabled the restoration of 450 acres within the Slough. It also created much-needed jobs in a state with high unemployment.  The project also benefits local recreational businesses like kayak outfitters, which depend on the habitats that make Elkhorn Slough a tourist destination.

Elkorn Slough
Contractors use an impact hammer to install the underwater sill at the Elkhorn Slough project.