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New detailed analysis of bathymetric digital elevation models (DEMs) spanning much of San Francisco Bay reveals a net loss of sediment since the 1980s, highlighting the need for ongoing study of the natural and human-caused changes in sediment volume in the Bay.

Map of San Francisco Bay showing rates of bathymetric change from 1980s to 2010s
Map showing rates of bathymetric change in San Francisco Bay from the 1980s to 2010s, from the study Sediment Deposition, Erosion, and Bathymetric Change in San Francisco, California, 1971-1990 and 1999-2020

This study, which analyzes DEMs created from bathymetric surveys collected in the 1980s and 2010s, helps researchers better understand how the bay has responded to changes such as sea-level rise and variations in sediment supply from the Sacramento-San Joaquin Delta and local tributaries. 

In addition to sediment erosion and deposition from natural processes, San Francisco Bay is directly impacted by human activities such as dredging channels, sediment extraction for development, sand mining, and dredged sediment disposal. 

For decades, USGS has developed and interpreted historical bathymetric digital elevation models of San Francisco Bay. Bathymetric change analyses document historical patterns of sediment deposition and erosion, providing valuable insight into the sediment dynamics of coastal systems, including pathways of sediment and sediment-bound contaminants.

The study—which covers about 80% of the Bay—found that the rate of net sediment loss in areas surveyed decreased from more than 6 million cubic meters per year (Mm3/yr) for the 1950s to 1980s to less than 1.5 Mm3/yr for the 1980s to 2010s, indicating that the delivery, transport, deposition, and erosion of sediment in the Bay has changed during those intervals. Additional research is needed to fully understand the causes for these changes and to enable accurate forecasts of future change. 

“The study's results can inform the creation of a new, system-wide sediment budget for the bay—an accounting of the amount of sediment that enters, is stored, and exits a system over time,” said USGS Geographer Theresa Fregoso, lead author of the study. “It can help researchers understand how sediment moves through a system, and can be used to manage sediment-related issues, such as shoreline erosion and habitat restoration.” 

The authors note that a potentially useful future analysis could link sediment losses and gains in San Francisco Bay with changes on land, including alterations of the shoreline and restoration or loss of wetlands. 

The study's findings emphasize the importance of ongoing monitoring efforts to understand how the bay is changing over time. As sea levels continue to rise, the bay will face increasing challenges related to coastal flooding and erosion. By tracking changes in sediment supply and distribution, researchers can better forecast how the bay will respond to these challenges and develop effective management strategies.

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