IMPERIAL BEACH, Calif. &
Along the U.S.-Mexico border, the fragile Tijuana Estuary is clinging to life as one of the last vestiges of undeveloped California coast, where marsh grasses sway gently in the breeze and rare birds stop to nest.
But just across the border is another landscape: old tires, plastic bottles, raw sewage and vast amounts of filthy sediment, all of it threatening to wash across the divide and spoil one of California's few surviving coastal wetlands.
Conservationists are struggling to protect the teeming estuary from the loose soil, trash and pollutants that have sometimes spilled downstream into the nature preserve from shantytowns in rapidly growing Tijuana.
With more than 90 percent of California's wetlands already lost to urban encroachment, "we're desperate to save every little scrap of coastal salt marsh we can," said Jeff Crooks, a research coordinator at the estuary.
At 2,500 acres, the estuary is three times the size of New York's Central Park, and for years it has remained largely undisturbed despite being encircled by development on both sides of the border.
This year, flash floods have washed silt and garbage from Tijuana into three catch basins on the American side built to trap debris before it seeps into the federally protected estuary.
The basins can store 60,000 cubic yards of debris &
enough to fill 6,000 dump trucks. But the pits are nearly full, and any more rain could cause the debris to overflow into the estuary.
"This area is under constant threat," said Oscar Romo, a coastal coordinator with the National Oceanic and Atmospheric Administration. "It's essential to keep it alive, to keep it in good shape."
The estuary, largely managed by the California and U.S. governments, was designated a "wetland of international importance" in 2005.
The pollution problem has grown more urgent in recent years with the rapid growth of Tijuana, a city of at least 1.3 million people that draws an average of 80,000 new residents each year.
Romo leads a cross-border effort to clean up a section of Tijuana's Los Laureles Canyon, a dirt-road community of 800 shanties built on steep slopes using scrap metal and used tires. When it rains, sediment and other debris flow across the border into the catch basins.
The sediment poses perhaps the most serious threat to the estuary. During the winter of 2004-2005, silt and sand from the canyon burst through the basins and buried 18 acres of salt marsh.
"The sediment problem is something we battle with every rainstorm," estuary manager Clay Phillips said. "I used to love the rain, but now I cringe and think, 'Are we going to lose marshland?'"
Romo's work is one of several conservation projects targeting the estuary. Other efforts are focused on restoring 250 acres of sediment-filled marshland and removing the invasive tamarisk shrub that competes with native plants.
Last month, Romo and a team of students from the University of California, San Diego, where he teaches urban planning, helped residents build roads that trap water in the ground instead of creating runoff that ends up in the estuary. It's a slow process. Only about a third of the 100,000 concrete blocks needed for the new pavement have been installed so far.
Many Tijuana residents were skeptical about the cross-border cleanup, which is funded by the U.S. and Mexican governments and a private foundation. To ease distrust, Romo holds workshops to teach residents about their environmental footprint.
"The people are part of this transformation," said Delia Castellanos, an environmental planner with Tijuana's Municipal Planning Institute.
Some scientists say the cleanup addresses only a small part of the problem because most of the polluted water infiltrating the estuary comes from the area's main watershed, three-quarters of which is in Mexico.
But if the estuary is not protected, experts worry about the potential loss of plant and wildlife diversity. Sand dollars and steelhead trout were once abundant in the estuary; now they have all but disappeared.
Joy Zedler, a botanist at the University of Wisconsin who has done research in the reserve since the 1970s, found last year that exotic plants were crowding out native ones, changing the ecosystem and altering the food chain.
Other researchers such as John Callaway of the University of San Francisco, who has studied the estuary for a decade, worry that important species could be lost if pollution isn't curbed.
"As we lose salt marsh," he said, "it's not just the vegetation that we lose, but all the animals that go with it."
Conservationists struggle to save wetland
IMPERIAL BEACH, Calif. &