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Map of the Sacramento-San Joaquin Delta and watershed identifying the eight major rivers that flow through the California Central Valley and enter the San Francisco Estuary as Delta outflow. The approximate location of the Central Valley Project's Shasta Dam, part of the largest reservoir in the watershed, is also shown.
Contexts in source publication
Context 1
... estuary's upstream watershed is characterized by large natural variations in annual precipitation and runoff, with peak annual runoff more than two times the average annual runoff. Annual streamflow volume in eight major rivers in the watershed (Sacramento, Feather, Yuba, American, San Joaquin, Stanislaus, Tuolumne, and Merced) (Figure 2), correcting for the effects of reservoirs and diversions and termed "unimpaired" runoff, is reported as the Eight River Index for measuring and comparing climatic conditions among years. Over WYs 1906-2017, the peak annual runoff from the Sacramento and San Joaquin River basins was 52.7 million acre-feet (maf); over this same period, mean annual runoff was 23.7 maf, and drought year runoff was often less than 10 maf (CDEC 2018 accounting of antecedent runoff conditions, are the basis for classifying water years into five categories: wet, above normal, below normal, dry, and critical. ...
Context 2
... estuary's upstream watershed is characterized by large natural variations in annual precipitation and runoff, with peak annual runoff more than two times the average annual runoff. Annual streamflow volume in eight major rivers in the watershed (Sacramento, Feather, Yuba, American, San Joaquin, Stanislaus, Tuolumne, and Merced) (Figure 2), correcting for the effects of reservoirs and diversions and termed "unimpaired" runoff, is reported as the Eight River Index for measuring and comparing climatic conditions among years. Over WYs 1906-2017, the peak annual runoff from the Sacramento and San Joaquin River basins was 52.7 million acre-feet (maf); over this same period, mean annual runoff was 23.7 maf, and drought year runoff was often less than 10 maf (CDEC 2018 accounting of antecedent runoff conditions, are the basis for classifying water years into five categories: wet, above normal, below normal, dry, and critical. ...
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Citations
Salinity, by definition, is the saltiness or amount of salt dissolved in a body of water. Salinity levels in Sacramento-San Joaquin Delta water have far-reaching impacts, affecting municipal, industrial, agricultural, and ecological water uses. Seasonal and daily salinity levels are driven by the amount of fresh water and ocean tides flowing into and through the Delta. Climate, hydrology, and human actions (drivers include tidal forces, rainfall, agricultural runoff , water diversions, and reservoir operations) also affect salinity.
Accurate estimates of freshwater flow to the San Francisco Estuary are important in successfully regulating this water body, in protecting its beneficial uses, and in accurately modeling its hydrodynamic and water-quality transport regime. For regulatory purposes, freshwater flow to the estuary is not directly measured; rather, it is estimated from a daily balance of upstream Delta inflows, exports, and in-Delta water use termed the net Delta outflow index (NDOI). Field research in the 1960s indicated that NDOI estimates are biased low in summer–fall and biased high in winter–spring as a result of conflating Delta island evapotranspiration estimates with the sum of ungauged hydrologic interactions between channels and islands referred to as net channel depletions. In this work, we employed a 50-year observed salinity record along with gauged tidal flows and an ensemble of five empirical flow-salinity (X2) models to test whether a seasonal bias in Delta outflow estimates could be inferred. We accomplished this objective by conducting statistical analyses and evaluating whether model skill could be improved through seasonal NDOI flow adjustments. Assuming that model residuals are associated with channel depletion uncertainty, our findings corroborate the 1960s research and suggest that channel depletions are biased low in winter months (i.e., NDOI is biased high) and biased high in late summer and early fall months (i.e., NDOI is biased low). The magnitude of seasonal bias, which can reach 1,000 cfs, is a small percentage of typical winter outflow but represents a significant percentage of typical summer outflow. Our findings were derived from five independently developed models, and are consistent with the physical understanding of water exchanges on the islands. This work provides motivation for improved characterization of these exchanges to improve Delta outflow estimates, particularly during drought periods when water supplies are scarce and must be carefully managed.
Climate change-driven sea level rise and altered precipitation regimes are predicted to alter patterns of salt intrusion within the San Francisco Estuary. A central question is: Can we use existing knowledge and future projections to predict and manage the anticipated ecological impacts? This was the subject of a 2018 symposium entitled “Ecological and Physiological Impacts of Salinization of Aquatic Systems from Human Activities.” The symposium brought together an inter-disciplinary group of scientists and researchers, resource managers, and policy-makers. Here, we summarize and review the presentations and discussions that arose during the symposium. From a historical perspective, salt intrusion has changed substantially over the past 10,000 years as a result of changing climate patterns, with additional shifts from recent anthropogenic effects. Current salinity patterns in the San Francisco Estuary are driven by a suite of hydrodynamic processes within the given contexts of water management and geography. Based on climate projections for the coming century, significant changes are expected in the processes that determine the spatial and temporal patterns of salinity. Given that native species—including fishes such as the Delta Smelt and Sacramento Splittail—track favorable habitats, exhibit physiological acclimation, and can adaptively evolve, we present a framework for assessing their vulnerability to altered salinity in the San Francisco Estuary. We then present a range of regulatory and structural management tools that are available to control patterns of salinity within the San Francisco Estuary. Finally, we identify major research priorities that can help fill critical gaps in our knowledge about future salinity patterns and the consequences of climate change and sea level rise. These research projects will be most effective with strong linkages and communication between scientists and researchers, resource managers, and policy-makers.
Invasive species have many detrimental ecological and socio-economic effects. However, invasive species can also provide novel habitat for native species. The growing rate of biological invasions world-wide presents an urgent dilemma: how can natural resource managers minimize negative effects of invasive species without depleting native taxa that have come to rely on them? Adaptive management can provide a means to address this dilemma when invasive species management plans are crafted in novel environments. We present a case study of research in support of adaptive management that considers the role of invasive water hyacinth (Eichhornia crassipes [Mart.] Solms [Pontederiaceae]) management, using herbicides, in aquatic food web functioning in the Sacramento–San Joaquin River Delta of California, USA (the “Delta”). We hypothesized that herbicide applications under current management protocols would reduce the abundance and diversity of aquatic invertebrates because they would alter both structural and biological habitat. Using a Before, After, Control, Intervention (BACI) experiment, we sampled invertebrates per gram plant biomass before and 4 weeks after glyphosate applications in treated and untreated locations. There was more plant biomass in the late-season samples because dead, dying, and living plant materials were compacted. However, there were no detectable differences between control and treated sites — or for samples before versus after the treatment date—for invertebrate abundance, species richness, or evenness. This case study demonstrates that even decaying water hyacinth serves as habitat for invertebrates that may be forage for Delta fishes. We concluded that current management practices using glyphosate do not affect invertebrate abundance during a month-long period of weed decay. These results provide valuable feedback for the “evaluate and respond” component of the adaptive management process for water hyacinth control, and demonstrate how managers globally can and should consider potential food web effects in the course of their invasive species management efforts.
