In 2014, California buried an abandoned airfield under 5.8 million cubic yards of harbor mud, then opened 998 acres to the tides

There is a stretch of shoreline in Marin County, California, that used to be a runway. Today, herons stand where fighter planes once landed. It took engineers more than ten years, millions of tons of harbor mud, and one deliberate breach in the levee to make this happen. The story matters beyond California. A peer-reviewed study published in PLOS ONE found that West Coast estuaries in the United States have lost about 85 percent of their original tidal wetlands to development and diking, with San Francisco Bay among the hardest hit. Hamilton is one way of clawing a little bit of that loss back.

From marshland to military runway, and back again

The site used to be a typical tidal marsh at the fringe of San Francisco Bay. The land was first diked off around the turn of the 20th century to make way for farming, and the U.S. Army later drained it further in 1930, constructed dikes to prevent tidal flooding, and built an airport known as the Hamilton Army Airfield, according to the project history recorded by Environmental Science Associates, the design consultant responsible for its restoration. The base operated for many years before being closed in 1988.

Cutting a wetland off from tidal flooding does weird things to land: without new sediment flowing in, the ground sinks. By the time the restoration began, engineers with the Army Corps' Engineering With Nature program noted that the old airfield had been buried about two meters beneath the surrounding marsh. If the levee had simply broken at that point, the site would have been flooded too deeply for marsh plants to grow. The ground had to be rebuilt first. That's precisely what the dredged sediment accomplished: raising the sunken site by roughly two meters, back up to the elevation the surrounding marsh sits at naturally, before the levee was breached.

Where do you get enough mud to raise 998 acres?

The solution was lying at the bottom of a nearby shipping channel. Meanwhile, the Port of Oakland was expanding its harbor, producing massive amounts of clean sediment that had to go somewhere. The Army Corps of Engineers and the California State Coastal Conservancy joined forces: pump that dredged mud through pipelines to Hamilton and use it to rebuild the land. Corps records show that approximately 5.8 million cubic yards of dredged sediment were placed on the former airfield between 2007 and 2013, including 3.5 million cubic yards from the Port of Oakland's harbor-deepening project. Sediment was graded to return the site to near natural marsh elevation. A 2.7-mile stretch of the Bay Trail now runs along the edge of the site, giving visitors a way to walk past the marsh they helped fund.

The day the bay came back in

When the excavator cut into the outer levee on April 25, 2014, waters from San Francisco Bay poured into the project site for the first time in more than 100 years. The Corps confirmed the breach completed construction on the approximately 648-acre site. Nearly all of that land has turned into wetlands of tidal, brackish, and fresh waters. It was not an overnight job, and it took years for marsh plants, tidal channels, and wildlife to reestablish themselves.

Why the mud strategy is more than a one-off fix

Hamilton also speaks to a bigger issue: rising seas. A 2011 study titled ‘Evaluating Tidal Marsh Sustainability in the Face of Sea-Level Rise: A Hybrid Modeling Approach Applied to San Francisco Bay’ published in PLOS ONE examined whether the marshes of San Francisco Bay could keep up with sea-level rise on their own. The study's authors concluded that many marshes will struggle without more sediment and specifically recommended redistributing dredged material to raise low-lying land as one of the few workable strategies available to planners. Hamilton had already begun doing exactly that years before the paper's 2011 publication, making it a real-world proof of concept for a technique researchers were only then formally recommending.

New neighbors: rare birds and a very small mouse

The restored marsh was built for specific residents. The Army Corps says the site supports the California Ridgway’s rail, a shy marsh bird found almost nowhere else on Earth, and the salt marsh harvest mouse, a tiny rodent found only around San Francisco Bay. Both are classified as endangered species. When the 2014 breach occurred, the rail was still officially called the California clapper rail, and it was renamed by ornithologists later that year. Waterfowl, shorebirds, and native plants have also made a comeback as the site has matured. The Army Corps will continue to monitor the site until 2027, 13 years after the breach, to see if the marsh develops as planned and if invasive plants are kept in check.

A blueprint, not a one-time story

Hamilton was more than the restoration of one airfield. It showed that dredged sediment, normally regarded as waste, can be used to restore wetlands. That lesson is likely to be put to use again as the seas continue to rise and cities look for ways to protect their shorelines, in the Bay Area and far beyond.