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The position of the low salinity zone in the San Francisco Bay Delta--given its correlation with the abundance of several estuarine species--is used for water management in a system that supplies water to more than 20 million people and contains one of the most diverse ecosystems on the Pacific coast. This work consolidates legacy and modern salinity data to develop a reasonably complete daily record spanning nine decades. The position of the low salinity zone, which is effectively characterized by an empirical model that was developed to support data cleaning and filling, reveals statistically significant trends consistent with increasing water demands and introduction of upstream reservoirs, e.g., increasing salinity trends in wet months and decreasing salinity trends in dry months. Reservoir effects are particularly apparent in drier years, with greater seasonal variability in the early part of the record before major reservoirs operated in the watershed. These data provide a basis for further analysis of how and why the position of the estuary's low salinity zone has changed over time. [ABSTRACT FROM AUTHOR]