Quick Take:
Mill Creek had been blocked since early in the last century by a 12-foot-tall, 25-foot-wide dam a quarter mile upstream from its confluence with San Vicente Creek. In September 2021, that dam was removed, but not because of the territory above the dam—the real goal is to restore waters below it.
The story of removing Mill Creek’s dam is a story about the pieces that fit together to bring life and vitality to an ecosystem. Kevin Sweeney, a Bay Area native, tells this granular story.
Towering coastal redwoods, charismatic coho salmon, and Pacific giant salamanders are, of course, main characters in this story—it’s the Santa Cruz mountains. A watershed is being restored, and in this part of the world, our eyes initially gravitate upward. But the crux of this magical restoration work along San Vicente Creek lies below us. This is a story about the granular: boulders, cobbles, pebbles, and sand.
San Vicente Redwoods is an 8,500-acre property east of Davenport. Purchased in 2014, it is jointly owned and managed by Sempervirens Fund and three partners: Peninsula Open Space Trust (POST), Save the Redwoods League, and Land Trust of Santa Cruz County. The property is protected primarily to restore, preserve, and conserve redwood forests, and watershed restoration has been a key focus at the property’s southern edge, where San Vicente Creek emerges from the trees and makes its way out to Davenport Landing to meet the Pacific Ocean.
High in the San Vicente watershed, the soil rests on a narrow band of karst, a highly porous limestone eroded and dissolved into a patch of fissures and sinkholes. These features trap and hold water, cooling it underground before releasing it in the springs that ultimately become San Vicente Creek, and further upstream, Mill Creek.
This feature is unique. Cold, persistent streams are rare here—the gift of karst. And the water is clear. In a shallow stretch across a sandbar, I had to look twice to check where the water was before stepping across the creek. The ample shade, blocking solar radiation, also keeps the canyon cool.
And San Vicente Creek no longer has an estuary as it meets the Pacific. For the last 200 yards of its run to the sea, it flows through tunnels, a bore through bedrock under the railroad and a box culvert under Highway 1. Though an uninviting doorway, this holds an important advantage. Because it doesn’t meander, instead passing in a straight shot to the ocean, it is less likely to lose energy and be stopped by a sand bar.
Human Impacts Along the Creek
San Vicente Creek was changed utterly when the dam was built on Mill Creek. The dam trapped the boulders, cobblestones, woody debris, and other material that had previously been carried downstream. Without the boulders and other large objects to divert flow, there was nothing to slow the creek’s water. Fast winter and spring waters cut deeper into the creek; waters no longer spilled over onto the floodplain, because the flow, even in winter, was well below that plain. When the river bottom was cut down to bedrock, there was no way deep pools would develop, depriving large fish of essential safe harbor. What should have been a winding stream had become a raceway.
Since the purchase of San Vicente Redwoods, Sempervirens Fund and its partners have seen restoration of the creek as central to its work there. Getting the stream back to health was important to watershed health and to the health of species that range thousands of nautical miles.
The dam would need to come out.
A Restored Stronghold
The actual removal of Mill Creek’s dam had the feel of an overnight sensation that in fact comes after a decade of work. The dam was removed quickly, most of it in the course of a single day. It was, of course, a day that followed years of preparation and staging. The planning process was inclusive, thoughtful, and comprehensive, and the long list of precursor actions was checked off deliberately. And then the dam was gone.
Here, we return to the granular details. Behind the dam was a trove of the materials required for a healthy riverbed—such stuff as streams are made of.
Boulders and cobblestone sequestered for a century behind the dam will, with heavy rains, descend Mill Creek and flow into San Vicente Creek. They will wedge against banks, against roots, against each other. They will push up against the large wood structures so carefully placed. Again stuck in place—this time the right place—they will resist flow, shifting the creek’s direction, if only briefly. Natural dams will form, and the creek may start to resemble a beaver colony’s reach, with water now slower and deeper.
The gravel and sand from behind the dam will find their more natural angle of repose. With the boulders, cobbles, and woody materials in place, these lighter granules will no longer race through the stream but will be part of it. The slowed water will allow the material to settle, raising the river bottom. Sand bars and gravel beds will form, creating newly ideal conditions for Coho salmon to spawn. As the bed continues to rise, high flows will spill over the banks, reconnecting stream and floodplain. The water will find alternate paths of descent as side channels open and braid across one another.
It’s all in place.