Watershed Controls on the Development of Tidal Marshes in Tomales Bay, California

Claire Divola, Alexander R. Simms, & Joseph A. Carlin

In Preparation September 1, 2026, SCEC Contribution #15666

Tidal marsh evolution is governed by interactions among sediment supply, watershed characteristics, and sea-level change, yet the relative importance of these controls within individual estuaries remains poorly understood. We investigated the spatial and temporal development of tidal marshes throughout Tomales Bay, California, by integrating historical aerial imagery (1965–2022), drainage basin-scale geographic analyses, multiple linear regression and Akaike Information Criterion corrected for small sample size (AICc) model selection, and sediment-core chronologies constrained by radiocarbon (14C), 210Pb, and 137Cs dating. Twenty-four marshes were mapped and classified according to patterns of persistence, expansion, loss, and transitional change. The two bayhead delta systems exhibited the greatest marsh stability and expansion, whereas the smaller fringing marshes along the eastern and western shorelines experienced greater marsh loss. Multiple linear regression and AICc model selection identified drainage basin area as the strongest predictor of modern marsh area, while modern drainage basin vegetation composition and bedrock geology were less statistically significant predictors. These results indicate that watershed size, and its associated capacity to supply sediment, exerts the primary landscape-scale control on marsh development. Sediment-core chronologies indicate that much of the modern marsh landscape developed within the past several few centuries, with substantial marsh establishment and expansion occurring during or after the mid-nineteenth century. Marsh initiation began earlier at some locations, including Walker Creek by approximately 1750 CE , but many of the marshes present today developed during the period of intensified watershed disturbance following Euro-American settlement. Larger drainage basins supported more extensive marsh development, while contrasting patterns of marsh-edge loss between the eastern and western shores suggest that watershed lithology influences modern marsh change, with those on the eastern side experiencing more landward side loss on those on the western side experiencing more seaward side loss . Together, these findings indicate that regional sediment supply initiated widespread marsh formation, whereas local watershed characteristics governed subsequent marsh growth and evolution. This study demonstrates the importance of integrating modern landscape analyses with geological records to understand tidal marsh evolution and provides a framework for predicting the resilience of estuarine wetlands to future environmental change.

Citation
Divola, C., Simms, A. R., & Carlin, J. A. (2026). Watershed Controls on the Development of Tidal Marshes in Tomales Bay, California. Estuarine, Coastal, and Shelf Science, (in preparation).