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Late Holocene Marsh Expansion in Southern San Francisco Bay, California
Citation:
WATSON, E. B. AND R. Byrne. Late Holocene Marsh Expansion in Southern San Francisco Bay, California. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 36(3):643-653, (2013).
Impact/Purpose:
Recent studies of tidal marsh geology and developmental history have established that in many locations, such as along tributaries of Chesapeake Bay and at Plum Island Sound in Massachusetts, wetlands formed only recently. This pattern of recent formation has been attributed to land use changes and increased erosion rates related to European-American settlement. This suggests that tidal landscapes are not in equilibrium with environmental conditions, and therefore may be quite vulnerable to anthropogenic modifications to coastal watershed or salt marsh hydrology. In this study, we examined formation ages for tidal marsh in South San Francisco Bay, California, and related episodes of marsh formation to potential contributing factors in order to provide context to the large-scale environment restoration underway in the region. We found greater rates of marsh establishment occurred during European settlement, but also during late pre-historic times, when climatic conditions resulted in increased freshwater and sediment influxes. Overall, we caution against using early historic maps as ‘baseline’ target for restoration, as these maps imply that South San Francisco Bay supported a much larger extent of wetlands than existed in the region for most of the Late Holocene.
Description:
Currently, the largest tidal wetlands restoration project on the US Pacific Coast is being planned and implemented in southern San Francisco Bay; however, knowledge of baseline conditions of salt marsh extent in the region prior to European settlement is limited. Here, analysis of 24 sediment cores collected from ten intact southern San Francisco Bay tidal marshes were used to reconstruct spatio-temporal patterns of marsh expansion to provide historic context for current restoration efforts. A process-based marsh elevation simulation model was used to identify interactions between sediment supply, sea-level rise, and marsh formation rates. A distinct age gradient was found: expansion of marshes in the central portion of southern San Francisco Bay dated to 500 to 1500 calendar years before present, while expansion of marshes in southernmost San Francisco Bay dated to 200 to 700 calendar years before present. Thus, much of the tidal marsh area mapped by US Coast Survey during the 1853–1857 period were in fact not primeval tidal marshes that had persisted for millennia but were recently formed landscapes. Marsh expansion increased during the Little Ice Age, when freshwater inflow and sediment influx were higher than during the previous millennium, and also during settlement, when land use changes, such as introduction of livestock, increased watershed erosion, and sediment delivery.
