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Mice, Men and Marshes
Workers at the controls of big backhoes, bulldozers and dredges raced to finish a new slough channel this June before a small white bird brought its young to the mouth of San Lorenzo Creek for fishing lessons. They succeeded. During the short weeks while the least terns, an endangered species, teach their offspring to dive and forage just offshore, the heavy equipment being used to restore 172 acres of salt marsh on the San Leandro shoreline is moving on to tasks farther inland.
This major South Bay wetland restoration project has been quietly steaming ahead while Al Gore visited Sonoma Baylands and Governor Wilson accepted the deed to 10,000 acres of North Bay salt ponds amid media fanfare. Despite all the press attention to the latter two wetland restoration projects, the former is equally significant. The San Leandro project's primary purpose is to restore habitat for existing populations of endangered salt marsh harvest mice. But it started more than a decade ago as a small mitigation project for wetlands used for upland disposal of material dredged from the San Leandro Marina.
"Back in the 70s, the city never imagined wetland restoration as extensive as this," says Greg Mailho of the City of San Leandro, the project sponsor. "It just kind of mushroomed."
The restoration has several major components. They're cutting a new channel to bring Bay water and tides back into several diked-off marshes. The new channel follows the old Roberts Landing Slough - where turn-of-the-century barges laden with local fruits and vegetables once travelled - except for at its mouth. "The delta and sandy beach that have built up at the mouth of San Lorenzo Creek have blocked off the mouth of the old channel," says hydrologist Bob Coats of Philip Williams & Associates. "Since we can't reopen the old route, we're following the new natural tendency." That tendency brings the slough into the creek rather than straight out to the Bay (see map).
Making sure there was enough scouring action in the slough bed to keep the new mouth open was one of the design challenges before Coats. To address it, he used a mathematical model called ESTFLO, which calculates energy, momentum and continuity in estuarine tidal flows. He used another model to estimate how many tide gates in the Bay shoreline levee he'd need to achieve what engineers call "muted" tidal action in the North Marsh - another component of the project.
"The land is subsided," explains Gary Oates of Environmental Science Associates. "If we just punched a hole in the levee, we'd have water inside all the time, and we'd drown the mice. But we designed it with Bob Coats to have multiple culverts, only one of which lets Bay water in, but all four of which let Bay water out."
The introduction of muted tidal action will help restore areas of pickleweed - prime salt marsh harvest mouse habitat - and Oates says workers are stockpiling pickleweed from excavations so they can spread it on five new islands. The islands will offer upland mouse refuge during high tides. "They're built like a sandwich," says Oates, with material from the main slough channel excavation at the base, from the city's dredged material disposal site in the middle, and from the smaller excavations - the pickleweed stockpile - on the top.
Beyond the North Marsh, the project will also restore tidal action to the East Marsh and the Bunker marsh - named after the ruins of ammunition bunkers from Trojan Powderworks. All in all, Coats says the restoration will create a good habitat mosaic, especially if a 106-acre complementary wetland re-hab project on its northeast border is added. This second project, planned mitigation for an environmentally controversial Citation Homes Central development, still hasn't received its final permit.
Gary Oates points out how well the City of Leandro's project showcases new tools for wetland restoration. The project was one of the first uses of the S.F. Regional Board's new guidelines for assessing suitability of dredged sediments for wetland construction. Oates found the criteria "useful." Before the criteria, he says, making the call on sediment suitability was pretty much left up to individual interpretation.
Oates says the project also demonstrates multiple beneficial reuse options for as much as 10,000 cubic yards of dredged material. The material - dredged up every 4-5 years from the marina and placed at a site adjacent to restoration area - is not only being used to build the islands but also being sold for sanitary landfill cover elsewhere. And in between dredgings, engineers plan to let 3-6 inches of bay water onto the disposal site to enhance the kind of seasonal ponding favored by shorebirds.
Contact: Gary Oates (415)896-5900; Bob Coats (415)981-8363; Greg Mailho (510)577-3481
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