San Francisco Estuary and Watershed Science 

About: San Francisco Estuary and Watershed Science is an academic journal published by eScholarship Publishing, University of California. The journal publishes majorly in the area(s): Estuary & Delta. It has an ISSN identifier of 1546-2366. It is also open access. Over the lifetime, 338 publications have been published receiving 6121 citations. The journal is also known as: SFEWS.

Critical Assessment of the Delta Smelt Population in the San Francisco Estuary, California

Abstract: The delta smelt (Hypomesus transpacificus) is a small and relatively obscure fish that has recently risen to become a major focus of environmental concern in California. It was formally abundant in the low-salinity and freshwater habitats of the northeastern San Francisco Estuary, but is now listed as threatened under the Federal and California State Endangered Species Acts. In the decade following the listings scientific understanding has increased substantially, yet several key aspects of its biology and ecological relationships within the highly urbanized estuary remain uncertain. A key area of controversy centers on impacts to delta smelt associated with exporting large volumes of freshwater from the estuary to supply California’s significant agricultural and urban water demands. The lack of appropriate data, however, impedes efforts to resolve these issues and develop sound management and restoration alternatives. Delta smelt has an unusual life history strategy relative to many fishes. Some aspects of its biology are similar to other coastal fishes, particularly salmonids. Smelts in the genus, Hypomesus, occur throughout the Pacific Rim, have variable life history strategies, and are able to adapt rapidly to local environments. By comparison, delta smelt has a tiny geographic range being confined to a thin margin of low salinity habitat in the estuary. It primarily lives only a year, has relatively low fecundity, and pelagic larvae; life history attributes that are unusual when compared with many fishes worldwide. A small proportion of delta smelt lives two years. These individuals are relatively highly fecund but are so few in number that their reproductive contribution only may be of benefit to the population after years of extremely poor spawning success and survival. Provisioning of reproductive effort by these older fish may reflect a bet-hedging tactic to insure population persistence. Overall, the population persists by maximizing growth, survival, and reproductive success on an annual basis despite an array of limiting factors that can occur at specific times and locations. Variability in spawning success and larval survival is induced by climate and other environmental and anthropogenic factors that operate between winter and mid-summer. However, spawning microhabitats with egg deposition have not been discovered. Spawning success appears to be timed to lunar periods within a water temperature range of about 15 to 20°C. Longer spawning seasons in cooler years can produce more cohorts and on average higher numbers of adult delta smelt. Cohorts spaced in time have different probabilities of encountering various sources of mortality, including entrainment in freshwater export operations, pulses of toxic pesticides, food shortages and predation by exotic species. Density dependence may provide an upper limit on the numbers of juvenile delta smelt surviving to the adult stage. This may occur during late summer in years when juvenile abundance is high relative to habitat carrying capacity. Factors defining the carrying capacity for juvenile delta smelt are unknown, but may include a shrinking volume of physically suitable habitat combined with a high density of competing planktivorous fishes during late summer and fall. Understanding the relative importance of anthropogenic effects on the population can be improved through better estimates of abundance and measurements of potentially limiting processes. There is little information on losses of larval delta smelt (less than 20 mm fork length, FL) to the export facilities. Use of a population model suggests that water export operations can impact the abundance of post-larval (about 20 mm FL) delta smelt, but these effects may not reflect on adult abundance due to other processes operating in the intervening period. Effects from changes to the estuarine food web by exotic species and toxic chemicals occur but measuring their influence on population abundance is difficult. Although delta smelt recently performed well enough to meet the current restoration criteria, analyses presented here suggest that there is still a high probability that the population will decline in the near future; the most recent abundance index (2004) is the lowest on record. Overall, the limited distribution, short life span and low reproductive capacity, as well as relatively strict physical and feeding requirements, are indications that delta smelt is at risk to catastrophe in a fluctuating environment. Unfortunately, options for avoiding potential declines through management and restoration are currently limited by large gaps in knowledge. Monitoring of spring water temperatures, however, may provide a useful tool for determining when to reduce entrainment in water export facilities. Actions that target carrying capacity may ultimately provide the most benefit, but it is not clear how that can be achieved given the current state of knowledge, and the limited tools available for restoration. Overall, a better understanding of the life history, habitat requirements, and limiting factors will be essential for developing tools for management and restoration. Therefore, given the implications for managing California water supply and the current state of population abundance, a good investment would be to fill the critical data gaps outlined here through a comprehensive program of research.

Open Water Processes of the San Francisco Estuary: From Physical Forcing to Biological Responses

Abstract: This paper reviews the current state of knowledge of the open waters of the San Francisco Estuary. This estuary is well known for the extent to which it has been altered through loss of wetlands, changes in hydrography, and the introduction of chemical and biological contaminants. It is also one of the most studied estuaries in the world, with much of the recent research effort aimed at supporting restoration efforts. In this review I emphasize the conceptual foundations for our current understanding of estuarine dynamics, particularly those aspects relevant to restoration. Several themes run throughout this paper. First is the critical role physical dynamics play in setting the stage for chemical and biological responses. Physical forcing by the tides and by variation in freshwater input combine to control the movement of the salinity field, and to establish stratification, mixing, and dilution patterns throughout the estuary. Many aspects of estuarine dynamics respond to interannual variation in freshwater flow; in particular, abundance of several estuarine-dependent species of fish and shrimp varies positively with flow, although the mechanisms behind these relationships are largely unknown. The second theme is the importance of time scales in determining the degree of interaction between dynamic processes. Physical effects tend to dominate when they operate at shorter time scales than biological processes; when the two time scales are similar, important interactions can arise between physical and biological variability. These interactions can be seen, for example, in the response of phytoplankton blooms, with characteristic time scales of days, to stratification events occurring during neap tides. The third theme is the key role of introduced species in all estuarine habitats; particularly noteworthy are introduced waterweeds and fishes in the tidal freshwater reaches of the estuary, and introduced clams there and in brackish water. The final theme is the rather heterogeneous set of results from monitoring and research in the estuary. For example, some topics have been subjects of intense activity both in research and monitoring (e.g., physical dynamics of the upper estuary, phytoplankton blooms), while others have received little attention (e.g., microzooplankton). In addition, both research and monitoring have emphasized some regions of the estuary (e.g., the Sacramento-San Joaquin Delta) over others (e.g., San Pablo Bay). In addition, ecological modeling and synthesis has emphasized lower trophic levels over higher. Opportunities for restoration in the open waters of the estuary are somewhat limited by the lack of scientific basis for restoration, and the difficulty in detecting ecosystem responses in the context of high natural variability.

From Climate-change Spaghetti to Climate-change Distributions for 21st-Century California

Abstract: The uncertainties associated with climate-change projections for California are unlikely to disappear any time soon, and yet important long-term decisions will be needed to accommodate those potential changes. Projection uncertainties have typically been addressed by analysis of a few scenarios, chosen based on availability or to capture the extreme cases among available projections. However, by focusing on more common projections rather than the most extreme projections (using a new resampling method), new insights into current projections emerge: (1) uncertainties associated with future greenhouse-gas emissions are comparable with the differences among climate models, so that neither source of uncertainties should be neglected or underrepresented; (2) twenty-first century temperature projections spread more, overall, than do precipitation scenarios; (3) projections of extremely wet futures for California are true outliers among current projections; and (4) current projections that are warmest tend, overall, to yield a moderately drier California, while the cooler projections yield a somewhat wetter future. The resampling approach applied in this paper also provides a natural opportunity to objectively incorporate measures of model skill and the likelihoods of various emission scenarios into future assessments.

Trends in the Sediment Yield of the Sacramento River, California, 1957 - 2001

Abstract: Human activities within a watershed, such as agriculture, urbanization, and dam building, may affect the sediment yield from the watershed. Because the equilibrium geomorphic form of an estuary is dependent in part on the sediment supply from the watershed, anthropogenic activities within the watershed have the potential to affect estuary geomorphology. The Sacramento River drains the northern half of California’s Central Valley and is the primary source of sediment to San Francisco Bay. In this paper, it is shown that the delivery of suspended-sediment from the Sacramento River to San Francisco Bay has decreased by about one-half during the period 1957 to 2001. Many factors may be contributing to the trend in sediment yield, including the depletion of erodible sediment from hydraulic mining in the late 1800s, trapping of sediment in reservoirs, riverbank protection, altered land-uses (such as agriculture, grazing, urbanization, and logging), and levees. This finding has implications for planned tidal wetland restoration activities around San Francisco Bay, where an adequate sediment supply will be needed to build subsided areas to elevations typical of tidal wetlands as well as to keep pace with projected sea-level rise. In a broader context, the study underscores the need to address anthropogenic impacts on watershed sediment yield when considering actions such as restoration within downstream depositional areas.

Subsidence, Sea Level Rise, and Seismicity in the Sacramento–San Joaquin Delta - eScholarship

Abstract: Anthropogenic accommodation space, or that space in the Delta that lies below sea level and is filled neither with sediment nor water, serves as a useful measure of the regional consequences of Delta subsidence and sea level rise. Microbial oxidation and compaction of organic-rich soils due to farming activity is the primary cause of Delta subsidence. During the period 1900-2000, subsidence created approximately 2.5 billion cubic meters of anthropogenic accommodation space in the Delta. From 2000-2050, subsidence rates will slow due to depletion of organic material and better land use practices. However, by 2050 the Delta will contain more than 3 billion cubic meters of anthropogenic accommodation space due to continued subsidence and sea level rise. An Accommodation Space Index, which relates subaqueous accommodation space to anthropogenic accommodation space, provides an indicator of past and projected Delta conditions. While subsidence and sea level rise create increasing anthropogenic accommodation space in the Delta, they also lead to a regional increase in the forces that can cause levee failure. Although these forces take many forms, a Levee Force Index can be calculated that is a proxy for the cumulative forces acting on levees. The Levee Force Index increases significantly over the next 50 years demonstrating regional increases in the potential for island flooding. Based on continuing increases in the Levee Force Index and the Accommodation Space Index, and limited support for Delta levee upgrades, there will be a tendency for increases in and impacts of island flooding, with escalating costs for repairs. Additionally, there is a two-in-three chance that 100-year recurrence interval floods or earthquakes will cause catastrophic flooding and significant change in the Delta by 2050. Currently, the California Bay-Delta Authority has no overarching policy that addresses the consequences of, and potential responses to, gradual or abrupt landscape change in the Delta.