Dynamic changes in sand supply in San Francisco Bay

A New research The study, conducted by the University of Texas, the San Francisco Estuary Institute and the U.S. Geological Survey, examined how the supply of sand to San Francisco Bay shifted from a primarily riverine source to an offshore source as sea levels rose during the early Holocene and as human activities altered sediment delivery during the late Holocene.

The San Francisco Bay (the Bay) is a large, structurally controlled estuarine system that is heavily influenced by the bedrock structure of its estuary. The prevailing hypothesis about sand transport in the Bay system is that sediments, including sand-sized sediments, are carried by the Sacramento and San Joaquin Rivers into the Bay and transported to the Pacific Coast. In this study, researchers used a comprehensive sand source analysis to test this hypothesis in an effort to better understand the sources and transport pathways of sand within the Bay.

Analysis and dating of sediment samples from the bay and its smaller bays indicate that the sand sources have changed significantly over time, particularly during the Holocene, from 11,700 years ago to the present day.

During the early part of this interglacial period, ice sheet retreat caused a rapid and dramatic rise in sea level. Sediment supply from river sources could not keep up with sea level rise, so the Bay received sediment from both river and marine sources. Human changes to the landscape during the late Holocene also affected sediment supply: from the 1850s to the mid-20th century, hydraulic gold mining activities in the foothills of the Sierra Nevada produced large amounts of debris that poured into San Francisco Bay. In addition, during the same period, as much as 50% of the Central Valley drainage basin was impounded by reservoirs, which impeded sediment transport from upstream basins to the estuary.

Research suggests that most of the sand currently in the bay is a relic of a period of lower sea levels before the Holocene. More recent influxes of sand may have been imported from the Pacific coast through the bay’s entrance.

A key implication of this research is that sand sources to large structured estuaries like bays can undergo abrupt reversals. The findings highlight the need for accurate, comprehensive models of sand inputs and transport pathways to coastal systems. These models are essential for developing effective management strategies that balance the extraction of coastal sand resources for economic purposes with the need to maintain natural coastal environments.

Read the research, Reversal of estuarine sand supply during Holocene sea-level rise: a model of estuarine sand transport in a large structured estuary in San Francisco Bay, California, USApublished in Earth and Planetary Science Letters.