The Mill Creek dam.
(Courtesy Ian Bornarth)

Removal of 100-year-old dam in Santa Cruz Mountains a ‘win-win’ for coho salmon and Davenport residents

The Sempervirens Fund has secured a grant to remove a defunct dam in the mountains above Davenport. The removal will benefit Coho salmon spawning grounds in Mill Creek, as well as improve water quality for residents downstream.

The Sempervirens Fund announced today that it has secured funding and permission to tear down a dam that has been holding up Mill Creek for more than a century.

It’s a move that benefits both humans ... and fish — specifically the endangered coho salmon.

“The Mill Creek dam removal project is truly a win-win project as it will improve water quality both for the residents and the coho,” county supervisor Ryan Coonerty said in an announcement about the project.

    The dam, a moss-covered stone wall about 15 feet high and 30 feet wide, was originally constructed to support redwood logging around the 1910s. Since then, it has prevented erosion of rocks downstream while backing up sediment, resulting in a dearth of gravel and sands that salmon need to lay their eggs. The dam also prevents the fish from traveling further upstream, cutting off access to more of their native habitat.

    According to the National Oceanic and Atmospheric Administration, habitat degradation due to dams and culverts is one of the main contributors to the demise of many salmon and trout populations in the U.S., including those in Santa Cruz County.

    “One of the key features and benefits of removing the dam is the release of that sediment into the creek bed, which is going to generate really prime spawning habitat,” says Matthew Shaffer, communications director with the Sempervirens Fund.

    Mill Creek, is part of the San Vicente watershed, which drains about 14 square miles of the Santa Cruz Mountains and provides drinking water for the community of Davenport. According to the EPA, the Bonny Doon School, Bosch Baha’i School and the Santa Cruz Waldorf School also draw water from the San Vicente Creek system, totaling the small watershed’s service to at least 950 people.

    A granite gift

    The San Vicente Creek and its tributaries are unique in that they run over granite rock, which is uncommon along the Central Coast. In some locations, the creeks have hollowed out tunnels and caves in the bedrock, disappearing underground and reemerging downstream. This helps slow the flow and keep the waterways from drying up in summer, says Ian Rowbotham, land stewardship manager at the Sempervirens Fund. Restoring this watershed in particular, he adds, is essential so it can be relied upon by people and fish when water is scarce.

    The CEMEX cement plant, which owns water and infrastructure rights to the dam property, has given its permission to demolish the structure. Deconstruction is set to begin in late summer. Funding for the project is coming through a $550,000 from the Open Rivers Fund, a program of the Resources Legacy Fund, which is supported by the William and Flora Hewlett Foundation.

    A second dam higher up in Mill Creek will remain in place as it continues to serve as a backup water source for Davenport.

    Life on the edge

    The Santa Cruz Mountains are at the southernmost edge of where both coastal redwoods and coho salmon can be found. Both species are federally listed as endangered, while the steelhead trout, another beneficiary of the dam removal project, is listed as threatened.

    The local populations of redwoods and cohos represent a unique window for scientists to see into the future. By studying organisms that live at the extreme ends of their current range, researchers can better predict how other populations will fare as Earth’s climate continues to shift.

    California’s intense 2020 fire season was a reminder that climate change is already affecting our local ecosystems. Portions of the San Vicente watershed, including stretches along Mill Creek, were burned in the CZU Lightning Complex fire last August, and many redwoods were lost further north. The salmon and steelhead trout that survived the fires are facing new challenges as they suffer the effects of excess runoff and an unusually dry winter season.

    Local and state agencies haven’t conducted robust fish count surveys for the past couple of years, but as part of the dam removal project the Sempervirens Fund and its partners are looking to reinitiate efforts to keep track of the fish numbers.

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    “If you give up on the outer limits of a species’ range, you may miss opportunities to see what’s working for them as we see these shifts,” Rowbotham says.

    They are also discussing potential research projects with San Jose State and UCLA, specifically on the use of environmental DNA, or eDNA, which allows researchers to determine how many salmon are in a stream by measuring the DNA the fish leave behind in the water.