A Comparison of Outflow and Salt Intrusion in the Pre-Development and Contemporary San Francisco Estuary

Abstract

The San Francisco Estuary and its upstream watershed have been highly altered by human development following the California Gold Rush in the mid-19th century. In this paper, we explore the inter-and intra-annual variability of freshwater flow to this estuary and the resulting salt intrusion under scenarios that represent pre-development and contemporary conditions. To place this comparison in context with the advent of systematic and accurate flow and salinity measurements in the estuary, we consider an additional "pre-project" scenario that represents early 20th-century water management (circa 1920), after major flood control and reclamation but before the introduction of large water storage, diversion, and export operations. We use an observed climate record that spans 82 years to compare freshwater flow associated with the scenarios' landscape and water use characteristics. Using published relationships between flow and salt intrusion length developed from three-dimensional hydrodynamic modeling, we evaluate the effect of these flow alterations as well as estuarine geometry modifications and historically observed sea level rise on salt intrusion. We conclude that the predevelopment estuary exhibited a more seasonally variable salinity regime, resulting from a more variable inflow regime from the upstream watershed.

Did a Shifting Ecological Baseline Mask the Predatory Effect of Striped Bass on Delta Smelt?

“…The era from 1850 to 1920 was characterized by large-scale wetland conversion, predominantly to leveed farmland, and by major additions to the estuary of hydraulic mining sediment (Nichols et al 1986;Schoellhamer 2011;Whipple et al 2012;Gross et al 2018). The main effect of wetland conversion would have been to limit the connectivity between water and the estuary's once-expansive marsh−floodplain habitats (Whipple et al 2012;Andrews et al 2017).…”

“…The period from 1920 to 1950 ushered in the first major modifications to the timing and duration of the estuary's freshwater flow regime (Hutton et al 2017;; freshwater inflow magnitude had begun to be affected in the previous era (Gross et al 2018). This second era of change began with historic drought and culminated with Central Valley Project rim dams on the Sacramento and San Joaquin rivers.…”

Source Water Apportionment of a River Network: Comparing Field Isotopes to Hydrodynamically Modeled Tracers

“…Its major diversions of freshwater, mainly from the Sacramento River, provide irrigation for the state's thriving agricultural industry, as well as drinking water for its cities [25]. These large diversions from the Delta, supported by an extensive system of reservoirs, have altered transport and seasonal salt intrusion trends in the estuary [26,27]. The nutrient-rich soils in the off-channel habitat of the Delta have been predominantly converted to agricultural plots [10], while at the same time over 95% of floodplain and intertidal habitat [28] have been channelized, thereby eliminating shallow-water habitat from the ecosystem.…”

Tidally‐averaged flows in the interior Sacramento–San Joaquin River Delta: Trends and change attribution

“…Recent work and regulatory inter-est points to the need for understanding the nexus between interior flows, survival of aquatic species, and general ecosystem health (Monismith et al, 2014). The present work supports the development of such understanding by interpreting historical interior flow trends and identifying the underlying drivers of change.This work, analogous to prior studies(Gross et al, 2018;Hutton et al, 2017b;Knowles, 2002), focuses on average monthly flows and not on finer time-scale flows such as daily or subdaily tidal flows. Tidal variability is an important aspect of all Delta flows and of relevance to the ecosystem.…”

“…The baseline scenario utilized in the change attribution analysis represents a 1920 level of development, that is flow changes were calculated relative to this 1920-level baseline. Prior studies have quantified changes in Delta outflow that resulted from landscape alterations independent of climatic drivers(Gross, Hutton, & Draper, 2018;Hutton et al, 2017b;Knowles, 2002). Our work builds upon the methodologies adopted in these prior studies by focusing on tidally-averaged interior Delta flow changes (which have not been analysed in a comparable manner) and by considering additional drivers that are relevant to these interior flows, such as the gates and barriers used to modify in-Delta circulation patterns.Ecosystem restoration efforts in the Delta have focused on targets for freshwater outflows from the Delta to San Francisco Bay, primarily through the position the estuarine salinity gradient defined by the X2 position(Jassby et al, 1995).…”