Something strange happened in San Francisco Bay in the late 1990s: the water suddenly grew clearer.
The levels of mud swirling around in the bay dropped 40 percent between 1998 and 1999, and “they’ve remained low since,” according to U.S. Geological Survey researcher David Schoellhamer.
Without that mud, most of the region's tidal marshlands could disappear as sea levels rise, Bay Area and Canadian researchers wrote in a study published Wednesday in the PloS ONE journal.
Mud that flows down rivers into Suisun, San Pablo and San Francisco bays helps marshlands grow by sticking to shorelines during high tides.
“The muddy, sediment-rich waters of the bay come over the marsh plain with each tide,” said Point Reyes Bird Observatory biologist Julian Wood. “That sediment is deposited; it falls out of the water column and onto the marsh.”
The amount of mud in the bays has decreased largely due to dams, which block large quantities of sediment from flowing down the rivers, and dredging, which removes some of the mud from bay floors. In addition, a thick flow of mud that was kicked loose by miners during the gold rush ran out in the 1990s.
Meanwhile, urban development, salt mining and levee construction have destroyed most of the tidal marshes that once ringed the three bays. The marshes that remain help filter pollution out of stormwater, protect buildings and roads from storm surges and provide habitat critical for fish and birds.
“They’re important spawning grounds for fish, and juvenile fish, including herring and anchovies, will use tidal marsh habitat for foraging,” said Wood, one of the report's authors.
Climate change has raised the stakes for the marshlands, which could drown unless they grow fast enough to keep up with rising seas.
The study examined how marshes in the bays could be affected over the coming century under various sea level rise and sediment shortage scenarios. Under the worst-case scenario of low mud concentrations and a sea level rise of more than five feet, more than 90 percent of the remaining marshlands in the Bay Area would disappear, the researchers reported. But if sediment becomes plentiful and sea level rise is less than two feet, marshes could prosper and expand.
As part of the study, Lisa Schile, a wetland ecologist with the University of California, Berkeley, mapped areas around the bays where marshes could potentially grow and migrate upland as seas rise. She found that such places are in short supply because of an abundance of levees, buildings, roads and other waterfront infrastructure.
“There really isn’t a lot of area for marsh migration with sea level rise,” Schile said. “We’ve basically built straight up to the bay.”
That means that many of the marshes could drown unless levees and other waterfront structures are removed to provide space for them to grow and migrate upland, according to Schile. She said that could require the purchase of private land by public agencies.
To Amy Wutzel, a staff member who works on sediment issues at the California Coastal Conservancy, a state agency that works to protect and restore coastal areas, the study is "just further evidence that we need to better manage our sediments in San Francisco Bay.”
Mud that is dug up during dredging operations in shipping channels, marinas and ports in the bays should be used for habitat restoration projects — as it was in the wetlands at Hamilton Airfield near Novato — instead of being dumped at sea, Hutzel said.
The mud could also be placed in strategic locations near marshes, which would help them grow as seas rise, according to Hutzel. But she said figuring out exactly where to deposit the dredged sediment could take more years of study.
Watch a documentary created by the U.S. Geological Survey about its research into sediment flows in San Francisco Bay: