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A new study conducted by the University of Texas, the San Francisco Estuary Institute, and USGS investigates how sand supply in San Francisco Bay reversed from largely fluvial sources to offshore sources as sea level rose during the early Holocene, coupled with anthropogenic changes that altered sediment transport in the late Holocene.

San Francisco Bay (Bay) is a large structurally controlled estuarine system heavily influenced by bedrock structures at its mouth. The prevailing hypothesis for sand transport across the Bay system has been that sediment (including sand-sized sediment) carried by the Sacramento and San Joaquin Rivers is delivered to the bay and transported to the Pacific coast. In the study, the researchers used a comprehensive sand provenance analysis to test this hypothesis, seeking to better understand origins and transport pathways of sand within the Bay.

Analyzing and dating sediment samples from the Bay and its smaller embayments revealed a significant shift in sand sources over time, particularly in the Holocene epoch from 11,700 years ago to present.

During early stages of this interglacial epoch, retreating ice sheets led to rapid and significant sea-level rise. Sediment supply from fluvial sources couldn’t keep pace with sea-level rise, and the Bay consequently received sediment from both fluvial and marine sources. Anthropogenic changes to the landscape during the late Holocene also affected sediment supply: A massive influx of detritus derived from hydraulic gold mining activity in the Sierra Nevada foothills reached San Francisco Bay starting in the 1850s and into the mid twentieth century. Additionally, as much as 50 percent of the Central Valley watershed has been impounded behind reservoirs during this same period, blocking sediment transport from the upper watershed to the estuary.

The study suggests that most of the sand currently present in the Bay is a relic from periods when sea levels were lower, prior to the Holocene. More recent influxes of sand are likely imported from the Pacific coast through the Bay's mouth.

One of the study's key implications is the potential for abrupt reversals in sand sourcing within large structural estuaries like the Bay. The findings underscore the necessity for accurate and comprehensive models of sand inputs and transport pathways in coastal systems. Such models are essential for developing effective management strategies that balance the extraction of coastal sand resources for economic purposes with the need to sustain natural coastal environments. 

Read the study, Reversal in estuarine sand supply driven by Holocene sea level rise: A model for sand transport in large structural estuaries, San Francisco Bay, California, USA, in Earth and Planetary Science Letters.

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