*2.2. Pre-Development Conditions*

Prior to development of the Central Valley and the San Francisco Estuary, the Sacramento, San Joaquin, and other rivers that drain the region had insufficient capacity to carry peak wet season flows generated by precipitation and snowmelt runoff. Rivers overflowed their natural levees in most years and discharged into adjacent low-lying basins, thus attenuating runoff to the Delta. As these flood flows receded, the low-lying basins would partially drain back to the rivers through smaller channels and sloughs; however, the basins typically remained inundated through late summer [32,33] Seasonal overtopping of the pre-development levees supported inland marshes [24,34–36], while riparian forests existed on natural riverbanks [37] and grasslands interwoven with vernal pools and valley oaks extended from the floodplains to the tree-covered foothills [38–40]. Water use by natural vegetation [41], in combination with the annual cycle of flooding, reduced the amount of precipitation and snowmelt runoff that reached the Delta. As natural levees were raised and wetlands and riparian forests were drained and cleared, water use by agriculture replaced water use by native vegetation in the Central Valley and the Delta. Fox et al. [24] estimated that annual water use from the natural landscape was similar to that of the highly altered contemporary landscape, such that freshwater flow reaching the estuary (i.e., Delta outflow) was minimally changed. In contrast to the Central Valley and Delta, land use changes in the surrounding foothill and mountain watersheds have been relatively minor [24]. The remainder of this section reviews previous efforts to characterize pre-development conditions using tree-ring data and flow–salinity modeling approaches.
