**2. Background**

To provide background for this work, we present a brief overview of the study region's geographic setting followed by a review of the region's hydrologic and salinity conditions over the past millennia. This review differentiates between three periods: a "pre-

development" period, a "contemporary" period, and an "early development" period that bridges the pre-development and contemporary periods. We define the terminus of the pre-development period as water year (WY) 1850, which roughly aligns with the California Gold Rush and follows previous work [24–26] California water years run from 1 October through 30 September. Furthermore, we define the start of the contemporary period as WY 1912, a date that aligns with availability of Delta outflow estimates [27] but pre-dates the availability of systematic estuarine salinity measurements by about a decade [14]. By default, the intervening early development period spans six decades between WYs 1851 and 1911.
