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The sediment levels in San Francisco Bay are going down, leaving wetlands with less protection from rising sea levels.
Oil continues to cloud Gulf Coast waters, but on the other side of the country, scientists are studying a body of water with the opposite problem. San Francisco Bay is becoming clearer. And clearer water is not always good news.
Underneath the dock on Alcatraz Island, U.S. Geological Survey hydrologist David Schoellhamer is collecting samples. "What we're measuring here is suspended sediment," he says. He holds up a bottle of cloudy water that he just collected. "Think of it as the microscopic rocks that are floating in the water."
For Schoellhamer, the bottle holds important clues to the bay's history. About 10 years ago, he noticed something strange in these samples: The bay's water was becoming clearer, after more than a century of murkiness.
"Back in the 1800s ... during the Gold Rush, the gold miners used essentially fire hoses and water cannons to literally wash down mountainsides to extract the gold from the sediment," Schoellhamer explains.
The mining sent masses of dirt from the Sierra foothills into the watershed.
"About 250 million cubic meters of sediment deposited in the bay," Schoellhamer says. That's enough mud, rock and sand to fill 36 million dump trucks.
"It really threw everything out of whack, because there was so much extra sediment. Now we're actually coming toward a new equilibrium."
He says all the extra sediment has finally worked its way past the Golden Gate. The bay's water is about 30 percent clearer than it was 10 years ago. That could help some fish and other species, but it poses a new risk for the bay's shoreline.
Wetlands At The Front Line Of Defense
In a salt marsh off the northern end of the bay, Professor John Callaway of the University of San Francisco and researcher Evyan Borgnis are taking samples of their own. They pull up a thick aluminum tube they've sunk into the soggy ground.
This thick mud was once sediment out in the bay. Callaway says it's key to the wetland's survival. With every high tide, these marshes are built up by the sediment they trap from the water.
Less sediment in the bay could spell trouble if scientists' predictions about rising sea levels come to pass. These delicate tidal marshes could be inundated over the next century.
"The big question is: How much will it go up in the future?" Callaway says. "If it goes up to 8 to 10 millimeters a year, then I think many wetlands are unlikely to survive."
Wetlands are home to all sorts of plants and animals -- from egrets to harbor seals and fish -- but they also do another job: they protect shoreline development from storm surges.
"What wetlands do is they can absorb those peak storm surges, those waves that are rolling in," says Steve Goldbeck of the Bay Conservation and Development Commission. "When they hit the wetlands, it knocks down the waves' peaks."
Goldbeck says critical infrastructure like the Oakland and San Francisco airports are in the path of rising waters. That puts wetlands on the front line of defense.
With 80 percent of the bay's historic wetlands already destroyed, Goldbeck says restoration projects will be a vital tool to combat the rise in sea levels -- but that will require a lot of sediment.
Fortunately, millions of cubic yards of sediment are dredged in the bay to keep shipping channels clear. Goldbeck's agency is looking at how to recycle that material, now that sediment is becoming a valuable resource.
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