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No Drought of Dirt

by Joe Eaton

With its massive environmental and economic costs, it’s hard to see a bright side to the California drought. Consider mud, though. According to US Geological Survey scientist David Schoellhamer, the long dry spell may be giving tidal wetland restoration efforts an unexpected boost by promoting the buildup of sediment in the South Bay where former salt ponds await conversion to tidal marsh.

Since the Gold Rush, San Francisco Bay received sediment churned up by hydraulic mining in the Sacramento and San Joaquin watersheds. That pulse had mostly spent itself by the 1950s. With the Bay’s sediment supply limited, there’s concern that tidal plains will be unable to build up fast enough to keep pace with rising sea levels. Restoration planning has turned to reuse of dredged sediment, a complicated and costly process.

However, Schoellhamer’s data shows a recent increase in suspended sediment in Bay waters near the Dumbarton Bridge. He and his colleagues have deployed underwater monitors that use optical sensing to measure sediment concentration, bouncing infrared light off suspended particles every 15 minutes. For water year 2013-14, their data show concentrations at the Dumbarton double those of the previous 10 years, with levels last seen in the 1990s. They’ve also found mud overlying shell fragments on the bottom of the Bay and accumulating in backwater sloughs bordering tidal marsh and salt ponds. Sediment concentrations in the rest of the bay have not increased.

Restoration of Pond A21. Photo: Mark Holmes
Restoration of Pond A21.
Photo: Mark Holmes

Normally, Schoellhamer explains, winds and waves push sediment toward the south end of the Bay. But in years of normal precipitation and snowpack, spring freshwater flows flush salt water out of the South Bay, taking sediment with it. “At the Dumbarton, we have observed sediment actually being pulled out of the South Bay during spring freshets,” he says. With greatly reduced freshwater flows, that effect has been muted, resulting in more mud staying in the South Bay.

Schoellhamer says the net landward movement of sediment may increase the accretion of inorganic material on tidal marshes and former salt ponds. Other consequences include increased turbidity, which could limit the productivity of phytoplankton. He points out that USGS monitoring programs are detecting other drought-associated changes in the Bay, including record high temperatures and salinity.

Upstream reservoir management impacts freshwater flows and sediment loads, of course. “Reservoir operators trying to capture the snowmelt before the dry season reduce the spring freshet effect,” Schoellhamer adds. “It shows how connected the whole system is, from the Sierra to the reservoirs to San Francisco Bay to the South Bay.”

Contact David Schoellhamer: dschoell@usgs.gov

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