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Yearslong construction work to transform a 435-acre pond once used for salt harvesting into a tidal marsh is wrapping up this month just north of Mountain View’s Shoreline Park.
Crews have strengthened levees, extended a public trail and erected islands for wildlife. The four-year project is a small part of the largest wetland restoration effort on the West Coast.
All that’s left to do is connect the pond to the creeks flanking it to the east and west, which discharge into the Bay. Over the next few decades, the tides will come up those waterways, bringing sediment into the pond, and plants will establish themselves. The hope is that a healthy marsh will form.
“Nature (will) do the actual restoration,” said Dave Halsing, executive project manager of the South Bay Salt Pond Restoration Project.
Marshes were once the dominant habitat ringing the San Francisco Bay, but after a century of industrial salt harvesting, around 85% of that marshland has been lost, according to the National Marine Fisheries Service. In 2003, the federal government paid $100 million to buy 15,100 acres of land around the Bay — an area a little larger than the size of Manhattan — from Cargill Salt, the primary commercial operator of local salt ponds.
Marshes are Mother Nature’s sponge. They absorb a tremendous amount of water and tremendous amount of wave action. That sponge function is just so powerful for sea level rise.
Pat showalter, Mountain View City Council member
Since then, federal, state and local officials, along with environmental groups, have worked to rehabilitate about a third of the ponds. Some were converted into deep and shallow pools for waterfowl, while others were kept dry for species that have adapted to the barren salt flats, such as the federally-threatened snowy plover. But many, like the Mountain View pond, are being reverted back to tidal marshlands.
“We’re trying to put as much back as we can because it’s the most natural ecosystem for this part of the world,” Halsing said.
A number of native species rely on marshes, including the endangered Ridgway’s rail and the salt marsh harvest mouse. The tidal wetlands also sequester carbon and filter water, Halsing added.
They’re also highly effective at protecting against floods, said Mountain View City Council member Pat Showalter, a civil engineer who previously worked on the salt pond project management team at Valley Water.
“Marshes are Mother Nature’s sponge,” Showalter said. “They absorb a tremendous amount of water and tremendous amount of wave action. That sponge function is just so powerful for sea level rise.”
How a 435-acre pond was rehabilitated into a marsh
But restoring a marsh is no easy feat and the Mountain View pond, formally called pond A2W, required extensive work. One particular challenge was that the pond was full of 2- to 5-foot deep water throughout the entire project.
“This was one of the harder ones,” Halsing said.
The total dollar amount spent on construction alone hasn’t been finalized yet, but Halsing said that it’ll fall somewhere between $20 million and $25 million.
Before the work even started, it took around three years to accumulate the dirt needed for the project. Project managers sourced 180,000 cubic yards of dirt, primarily from local construction sites. A single cubic yard is about the size of a washing machine, Halsing said.
On the north levee that dikes the pond off from the Bay, crews removed a defunct water control tower and repaired the eroding levee, plugging the hole the crumbling structure had left when it was taken out.
The opposite side of the pond abuts Shoreline Park, which sits atop a closed landfill. To prevent future tidal fluctuations from eroding the landfill’s levee, project organizers and the city spent years planning a barrier that would safeguard the infrastructure.
“The design incorporated elements of nature-based solution concepts, with rocks buried beneath a soil layer, so the slope can be replanted, providing habitat for wildlife while preserving the natural look of the shoreline,” city spokesperson Lenka Wright said in an email.
Local nonprofit Save the Bay will spend the next year tending to the sloped habitat, also called an “ecotone” or a “habitat transition zone.” The organization has been a vocal supporter of the South Bay Salt Pond Restoration Project since its early days, said Save the Bay Executive Director David Lewis.
The nonprofit advocated for voters to pass the Bay Area-wide Measure AA in 2016. The parcel tax brings in around $25 million annually for wetland restoration, flood protection and habitat preservation. Funding from Measure AA was used to pay for some of the cost of construction at pond A2W, Halsing said.
More recently, Save the Bay helped reestablish the native vegetation across large swaths of the pond restoration project, Lewis said. In Menlo Park, where a complex of five ponds was rehabilitated, the nonprofit introduced native plant seed mixes to combat weeds and hired local farmers to help spread plant material. Similar techniques will be used on A2W’s sloped habitat, Lewis said. The nonprofit’s staff are also raising local vegetation, like gumplant and salt grass, in Save the Bay’s Peninsula-based nurseries to plant at the site.
Local flora provide cover to species living in the wetlands during high tides, Lewis said, and their root systems reduce erosion by holding soil together.
“They’re also sequestering carbon in the soil,” he added. “They don’t necessarily sequester as much as a redwood tree does, but they capture carbon from the air that otherwise wouldn’t be if it was just open dirt.”
On the south end of the pond, the sloped habitat that protects the landfill’s levee runs parallel to the Bay Trail which formerly dipped into an emergency flood channel near the eastern edge of the pond. Crews raised the path and installed a storm drain system below it.
“This low area previously served as an emergency overflow route to provide flood relief from the North Bayshore Area,” Wright said in an email. “The culvert system allows stormwater to drain, while a series of tide gates prevents bay water from flowing inland.”
On the pond’s east side, crews built a roughly one-mile extension of Stevens Creek Trail on top of the levee separating the pond from Stevens Creek. In order for water from the Bay to enter the pond, that levee has purposely been breached in one spot and another breach is expected to be created by the end of the month. Workers also constructed two armored bridges over the breach locations to create the trail extension, which leads to a lookout onto the Bay.
On the west side of the pond, the area was designed solely for wildlife. Using a system of conveyor belts, crews transported dirt to a spot around 100-feet into the wetland and built up five islands for animals. For now, birds will likely use the small islands for nesting and roosting, Halsing said.
But the islands are “going to play different ecological purposes over time,” he added. As the pond transforms more into a marsh over the years, they’ll become spots on which terrestrial wildlife can shelter during high tides.
The public won’t have access to this side of the pond.
“We have to strike that balance between giving people a place to go and recreate and get in touch with nature and do stuff, but also protecting the wildlife that needs some degree of privacy and distance from people,” Halsing said.
And the plan is to expand this dedicated wildlife area even more. To the west of A2W, water flows through the Mountain View Slough to the Bay and just beyond it lies a 275-acre pond called A1. Currently, levees separate the two ponds from the slough. Crews will breach the levee between the slough and A2W by the end of the month. Tidal fluctuations will slowly erode those openings allowing them to expand over decades.
Rehabilitation work on A1 won’t begin for years, Halsing said, but eventually the levee between that pond and the slough will be breached too, bringing in water from the Bay. The result will be “one big marsh,” Halsing said, with almost 700 acres of connected wetland.
“Fish will be able to go back and forth between the two ponds,” he said.
While construction work at A1 is still a few years out, crews have already started trucking in dirt. Just like A2W, the pond is already filled with water and birds can be seen floating around.
“It’s a very similar project,” Halsing said. “It’s going to be a tidal marsh. There’s going to be an ecotone against the southern side. There will be a short section of trail in one corner with a viewing area and there’ll be another viewing area inside of Shoreline Park, kind of on the Bay Trail, looking down into the pond.”
In February, project organizers plan to host an event open to the public to celebrate the completion of the work at pond A2W.
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Looking forward to the reopening of the Bay Trail and the new extension of Stevens Creek Trail. Please update with dates when those are learned. Also the event to celebrate this. Thanks.
This is such important work. Thank you very much for providing the details in your article. I am very happy to learn of these efforts.