BackgroundAccumulating evidence shows that the planet is warming as a response to human emissions of greenhouse gases. Strategies of adaptation to climate change will require quantitative projections of how altered regional patterns of temperature, precipitation and sea level could cascade to provoke local impacts such as modified water supplies, increasing risks of coastal flooding, and growing challenges to sustainability of native species.Methodology/Principal FindingsWe linked a series of models to investigate responses of California's San Francisco Estuary-Watershed (SFEW) system to two contrasting scenarios of climate change. Model outputs for scenarios of fast and moderate warming are presented as 2010–2099 projections of nine indicators of changing climate, hydrology and habitat quality. Trends of these indicators measure rates of: increasing air and water temperatures, salinity and sea level; decreasing precipitation, runoff, snowmelt contribution to runoff, and suspended sediment concentrations; and increasing frequency of extreme environmental conditions such as water temperatures and sea level beyond the ranges of historical observations.Conclusions/SignificanceMost of these environmental indicators change substantially over the 21st century, and many would present challenges to natural and managed systems. Adaptations to these changes will require flexible planning to cope with growing risks to humans and the challenges of meeting demands for fresh water and sustaining native biota. Programs of ecosystem rehabilitation and biodiversity conservation in coastal landscapes will be most likely to meet their objectives if they are designed from considerations that include: (1) an integrated perspective that river-estuary systems are influenced by effects of climate change operating on both watersheds and oceans; (2) varying sensitivity among environmental indicators to the uncertainty of future climates; (3) inevitability of biological community changes as responses to cumulative effects of climate change and other drivers of habitat transformations; and (4) anticipation and adaptation to the growing probability of ecosystem regime shifts.

The transition of flow between laterally confined channels and the unchannelized delta front controls the morphodynamic evolution of river deltas but has rarely been measured at the field scale. We quantify flow patterns and bathymetry that define the evolution of the subaqueous delta front on the Wax Lake Delta, a rapidly prograding delta in coastal Louisiana. A significant portion of flow (∼59%) departs the channel network over lateral channel margins as opposed to the downstream channel tips. Bathymetric surveys and remotely sensed estimates of flow direction allow spatial changes in flow velocity to be quantified and patterns of erosion and deposition to be estimated. Shallowing along channel margins produces spatial acceleration and erosion. Lateral spreading, deceleration, and deposition occur within three to eight channel widths outside of the channel margins. In interdistributary bays, the shape of each flow path is constrained by "nourishment boundaries" that separate the outflows from neighboring channels. Deposit elevation decreases with a basinward slope of 2.4 × 10 −4 with distance from a channel margin along any flow path, regardless of the channel or location that flow departed the network. Bathymetric depressions called "interdistributary troughs" form along nourishment boundaries where flow paths are the longest and deposit elevation is correspondingly low. We conclude that the deposit morphology exerts a strong control on bathymetric evolution and that interaction between neighboring channels and even neighboring deltas can influence delta front morphology.

Changes in water temperatures caused by climate change in California's Sacramento-San Joaquin Delta will affect the ecosystem through physiological rates of fishes and invertebrates. This study presents statistical models that can be used to forecast water temperature within the Delta as a response to atmospheric conditions. The daily average model performed well (R 2 values greater than 0.93 during verification periods) for all stations within the Delta and San Francisco Bay provided there was at least 1 year of calibration data. To provide long-term projections of Delta water temperature, we forced the model with downscaled data from climate scenarios. Based on these projections, the ecological implications for the delta smelt, a key species, were assessed based on temperature thresholds. The model forecasts increases in the number of days above temperatures causing high mortality (especially along the Sacramento River) and a shift in thermal conditions for spawning to earlier in the year.

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Climate change is driving rapid changes in environmental conditions and affecting population and species’ persistence across spatial and temporal scales. Integrating climate change assessments into biological resource management, such as conserving endangered species, is a substantial challenge, partly due to a mismatch between global climate forecasts and local or regional conservation planning. Here, we demonstrate how outputs of global climate change models can be downscaled to the watershed scale, and then coupled with ecophysiological metrics to assess climate change effects on organisms of conservation concern. We employed models to estimate future water temperatures (2010–2099) under several climate change scenarios within the large heterogeneous San Francisco Estuary. We then assessed the warming effects on the endangered, endemic Delta Smelt, Hypomesus transpacificus, by integrating localized projected water temperatures with thermal sensitivity metrics (tolerance, spawning and maturation windows, and sublethal stress thresholds) across life stages. Lethal temperatures occurred under several scenarios, but sublethal effects resulting from chronic stressful temperatures were more common across the estuary (median >60 days above threshold for >50% locations by the end of the century). Behavioral avoidance of such stressful temperatures would make a large portion of the potential range of Delta Smelt unavailable during the summer and fall. Since Delta Smelt are not likely to migrate to other estuaries, these changes are likely to result in substantial habitat compression. Additionally, the Delta Smelt maturation window was shortened by 18–85 days, revealing cumulative effects of stressful summer and fall temperatures with early initiation of spring spawning that may negatively impact fitness. Our findings highlight the value of integrating sublethal thresholds, life history, and in situ thermal heterogeneity into global change impact assessments. As downscaled climate models are becoming widely available, we conclude that similar assessments at management-relevant scales will improve the scientific basis for resource management decisions.

Shallow coastal regions are among the fastest evolving landscapes but are notoriously difficult to measure with high spatiotemporal resolution. Using Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) data, we demonstrate that high signal‐to‐noise L band synthetic aperture radar (SAR) can reveal subaqueous channel networks at the distal ends of river deltas. Using 27 UAVSAR images collected between 2009 and 2015 from the Wax Lake Delta in coastal Louisiana, USA, we show that under normal tidal conditions, planform geometry of the distributary channel network is frequently resolved in the UAVSAR images, including ~700 m of seaward network extension over 5 years for one channel. UAVSAR also reveals regions of subaerial and subaqueous vegetation, streaklines of biogenic surfactants, and what appear to be small distributary channels aliased by the survey grid, all illustrating the value of fine resolution, low noise, L band SAR for mapping the nearshore subaqueous delta channel network.

Coastal wetland systems are among the most dynamic landscapes on Earth's surface; however, interrelated processes create wetland platforms that are relatively constant in space and time. Theoretically, “stable” elevations should maintain themselves through time if the balance of processes creating that elevation remains unchanged. At Louisiana's prograding Wax Lake Delta, we measure landscape change between 2009 and 2013, quantifying volumetric changes to the delta, subaerial slope adjustment, and an equilibrium elevation of 0.56 m North American Vertical Datum of 1988 (where elevation change, on average, is zero), around which elevations fluctuate. We calculate a system average “equilibrium time scale” of 16 years to describe how long locations will take to approach the stable elevation. This time scale increases as a function of distance from channel edge. Peaks in elevation probability density functions can form for multiple reasons and do not require stability; e.g., peaks can temporarily form at elevations where elevation change rate is locally minimum.

River deltas form complex branching patterns that distribute sediment to the coastal sea. The routing and storage of this sediment in deltas are poorly understood. We present results of a 1‐month study of the sediment and water transport through two branches of the Wax Lake Delta on the coast of Louisiana. The two channels maintained near‐equal total partitioning of flow and sediment discharge. East Pass was narrower and had higher tidally averaged velocities, lower tidal velocity fluctuations, more sediment flux, and less alluvial bed cover than Main Pass. We connected these differences to small differences in the geometry of the channels and feedbacks between these differences. East Pass was slightly shorter than Main Pass, leading to a steeper mean free surface profile, yielding higher subtidal currents and advective sediment fluxes. Main Pass received the tides from Atchafalaya Bay earlier than East Pass, allowing tides to reach their junction earlier from Main Pass. This led to Main Pass capturing tidal prism from East Pass and higher tidal velocity fluctuations in Main Pass. These fluctuations caused larger flow convergences in Main Pass, pumping sediment out of East Pass on tidal timescales. Since East Pass had higher mean velocities and lower tidal fluctuations, it spent less time in flow regimes that would allow the formation of alluvial bed cover. By explaining these asymmetries with physical arguments, we showed how geometric differences drive asymmetries in branching systems. Barrier island inlets, coastal lagoons, and other systems may display similar behavior.