Floating wetlands sound like something straight out of a fairy tale, fanciful landscapes where the laws of nature are suspended. But these buoyant mats of peat and tall, spiky marsh plants called tules are very real. That said, floating wetlands do possess qualities that border on magic.
They historically broke off from marshes in the Sacramento-San Joaquin River Delta, and could be so big―up to several acres―that people call them floating islands. And they’re so sturdy that one even bore a herd of livestock to safety during California’s Great Flood of 1862, which filled the Central Valley to depths of 30 feet.
People can walk on floating wetlands too. “They’re squishy, and kind of bob up and down,” says Steven Deverel, principal hydrologist at the Davis-based consulting firm HydroFocus.
But, Deverel and other researchers say, the most wondrous thing about floating wetlands is their potential for restoring the Delta. According to new findings that are preliminary but promising, floating wetlands could help Delta smelt and other imperiled native fish.
One reason fish are in trouble in the Delta is that there isn’t enough for them to eat. “The Delta is often called a food desert,” Deverel says.
The Delta’s once-vast wetlands teemed with tiny creatures called zooplankton that fish grew fat on. But nearly all that tidal marsh is gone due to the region’s wholesale conversion to leveed islands for agriculture. Efforts to reestablish tidal marsh are underway but suitable sites are limited.
“There aren’t many places where you can put tidal wetlands in the central Delta,” Deverel says.
The problem is that islands there have undergone extreme soil loss, or subsidence, since being drained for farmland beginning in the 1800s. The central Delta’s soil is primarily peat, which is made of partly decomposed wetland plants. Exposure to air completes the decomposition, oxidizing layer after layer of soil away. Today these islands have sunk so far below the Delta’s waterways that the land surface can no longer reconnect with the tidal flows required to establish new marshes.
FOOD, SHELTER AND COOLER WATER FOR FISH
So Deverel and colleagues decided to see if floating wetlands, which are independent of land, could mimic the benefits of tidal marsh to fish. They got the idea by chance. In 2005, restoration practitioners brought dredge sediment to rebuild tidal marsh in Montezuma Wetlands. To their surprise, some of the dredge material went under the wetland’s peat soil, making it float. “It was accidental,” Deverel explains.
First the team wondered if they could similarly create floating wetlands on purpose. Then they learned that they naturally occur worldwide, including historically in the Delta. Using chunks of peat from wetlands “seemed simpler than trying to lift the peat soils from underneath,” Deverel says.
In 2019, the team dug peat blocks from restored marsh on Twitchell Island and installed them in above-ground swimming pools on Bouldin Island, which is in the central Delta and is owned by the Metropolitan Water District of Southern California.
The resulting floating wetlands are now spectacular, with tule stems waving above and roots extending below. Even better, the floating wetlands had a wealth of fish food. In just one year, zooplankton density in the floating wetlands was triple that in the river bordering the island, according to a study led by Deverel and scheduled to publish in the December issue of San Francisco Estuary and Watershed Science.
The tangle of roots under floating wetlands also provides nooks and crannies to shelter baby fish, giving them safe places to grow up. Floating wetlands shade the water too, cooling it by 2⁰C―enough to mean the difference between life and death for fish when temperatures spike in the Delta.
Now the team hopes to demonstrate the value of floating wetlands to fish on a larger scale. The pilot floating wetland is about 2,500 square feet, and Deverel envisions expanding that to as much as half an acre (nearly 22,000 square feet) in a wide channel, contained by a boom to keep it in place. The big question is whether they can get enough peat blocks to jumpstart it. Permitting could be a hurdle but Deverel is hopeful, noting that the Twitchell Island marsh that yielded their experimental peat blocks has completely regrown and that , between Twitchell and Sherman islands, there are 2,000 acres of restored marsh nearby.
“With a larger floating wetland, I think people could really see the habitat restoration and fish food benefits they provide,” Deverel says.
RAMPING UP PRODUCTION OF DELTA SMELT
Shawn Acuña, a fish ecologist at the Metropolitan Water District of Southern California, is already enthusiastic about the promise of floating wetlands. He’s part of a team working to bolster production of Delta smelt, which are extremely rare in the wild.
So far, state and federal agencies have reintroduced Delta smelt raised at the University of California, Davis Fish Conservation and Culture Laboratory. But this operation only has so much space and is labor intensive, making it a challenge to ramp up production. Agencies released about 90,000 Delta smelt this year, and models suggest it will take annual releases of several hundred thousand to restore these fish in the wild.
“Floating wetlands in ponds could be a lot cheaper to upscale,” Acuña says.
Over the last three years, Acuña and colleagues have raised Delta smelt in cages in two existing ponds on Bouldin Island. The ponds are in huge holes called borrow pits, where soil was excavated to maintain levees.
“Fish growing in the ponds did really well,” Acuña says.
Delta smelt survival in ponds was high and their weight increased by an average of one-third. Fatter fish make eggs with more yolk, which helps baby fish survive tough times. The latter came as a surprise to Department of Water Resources researchers because their caged fish, which were in a river, tended to lose weight. The difference is that ponds have a lot more fish food than the Delta’s rivers.
Floating wetlands could boost fish food in ponds even more, and the team worked out the nuts and bolts of testing this last spring. The set-up entails anchoring Delta smelt cages near floating wetlands, which are contained by a boom. Next year the team hopes to see if the tules will ramp up fish production.
“Ponds could potentially grow hundreds of thousands of fish,” Acuña says.
POTENTIAL HURDLES
Ponds could also have downsides for fish. In rivers, fish may be able to swim to more favorable waters. But in ponds, “they’re at the mercy of the environment,” Acuña says. Heat is likely to be the biggest problem, he adds, explaining that ponds can be much hotter than rivers in the Delta.
Soon the team will have the chance to test ways of optimizing ponds for Delta smelt. The last time workers dug borrow pits on Bouldin Island, they created six identical ponds for the researchers that will be perfect for experiments. Possibilities for cooling ponds besides floating wetlands include shade balls or pumping in groundwater. Shade balls are small spheres that help control water temperatures by blocking sunlight. Groundwater in the Delta is near the surface and generally chilly.
“I’m excited about floating wetlands,” Acuña says. “If we can get them to work, it will transform restoration in the Delta.”
