San Francisco Estuary and Watershed Science

Freshwater inflow is vital for the ecological health of estuaries. Understanding historical flow volume and timing is therefore essential for sustainable management and restoration of these environments. Using legacy hydrologic data—including riverine water-level measurements, watershed runoff estimates, and wetland reclamation records—we extended a monthly time-series of freshwater inflow to San Francisco Estuary by 6 decades, back to California’s Gold Rush era. This period marks the onset of significant anthropogenic modifications to the waterscape. Our analysis of the extended series, normalized to unimpaired runoff, reveals an increasing trend in systemwide water use that was preceded by a decline in the latter half of the 19th century. We hypothesize this decline resulted from reduced evapotranspiration as a result of vegetation removal and reduced overbank flows from levee construction. These findings align with earlier research that shows similarities between natural and contemporary long-term annual average inflow, comparing pre-development conditions to those of the early 20th century and today. Monthly flow trends, however, displayed more nuanced, season-specific effects of human modifications. Despite unusually wet hydrology during the reconstruction period, our findings comprise an important contribution to ongoing dialogue on ecosystem-restoration targets.