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San Francisco Bay’s Muddy Shallows Captured in New Documentary
Released: 6/23/2011 12:00:00 PM

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Jessica Lacy 1-click interview
Phone: 831-427-4720

Paul  Laustsen 1-click interview
Phone: 650-329-4046

MENLO PARK, Calif. – A new U.S. Geological Survey documentary, Turbid Bay: Sediment in Motion, describes ongoing research into the transport of sediment at the margins of San Francisco Bay. 

The film follows USGS Mendenhall Research Fellow Lissa MacVean and her USGS advisor Jessie Lacy who carefully gather information for a study that provides an important piece of a large ecosystem puzzle.  The study begins to address the question of how sediment is supplied to marshes by looking in detail at how bay sediment moves from areas that are always inundated, or subtidal, into intertidal mudflats that are alternately wet and dry.

“Some wetlands-restoration projects actually deposit sediment to bring the marsh plain elevation up to the appropriate level for plants,” said Lacy, a research oceanographer, “but it’s considered a much better option to rely on natural processes because that is a sustainable restoration. And relying on natural processes means assuming that the turbid waters of the bay will deposit enough sediment in the marsh to restore it.”

These transport processes are critical to the mudflats and marshes that ringed the Bay, historically. More than 80 percent of the tidal marshes that existed before the 1850s and the Gold Rush have been lost because of human activities, including diking, draining, and filling.

San Francisco Bay is a dynamic estuary where seawater entering through the Golden Gate mixes with freshwater from the San Joaquin and Sacramento Rivers and numerous local streams. Fringing the bay are tidal marshes that play an important role in bay ecosystems. The USGS provides science support to many of the local, state, and federal efforts to improve the health of the bay by restoring tens of thousands of acres of commercial salt ponds, diked agricultural acreage, and other lands to functioning tidal marsh and shallow ponds.

In the documentary MacVean, Lacy, and other members of the USGS team place instrumented platforms at four sites in San Pablo Bay, a northern embayment of San Francisco Bay. The four sites lie along a gradient from the 12-m deep channel in the southern part of San Pablo Bay to intertidal mudflats exposed to air at each low tide. Deployed for 6 weeks, the instruments gathered a massive amount of data, making measurements as fast as 10 times per second.

“We’re making field measurements of water velocities, salinities, and suspended sediment in order to determine exactly what’s controlling how sediment moves in really shallow environments in an estuary,” said MacVean.

Among other things, the data will show how sediment concentrations and water velocities change over a range of time scales—as the seasons change, when storms come and go, when it’s windy or not windy, through each tidal cycle, and during the passage of a single wave.

“An even shorter time scale is the turbulence time scale,” said Lacy. “Although waves pick the sediment up off the bed, it’s the turbulence—the tiny, random motions caused by the interaction of currents with the bed and wind with the water surface—that actually mixes sediment up through the water column. In previous experiments, mostly in deeper water, we’ve been able to resolve that combination of processes. In this experiment, we’ll be examining those processes right at this intertidal zone, where there are very few measurements.”

This type of data is essential for verifying the accuracy of sediment-transport models, which can then be used to predict the impacts of long-term processes such as marsh restoration and sea-level rise.

View the new documentary online. A companion article was printed in the April/May 2011 issue of Sound Waves, a newsletter about USGS coastal and ocean research.

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