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As temperatures rise, it is increasingly important to monitor changes in habitat conditions for aquatic species and mitigate emerging stressors. The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) instrument provides temperature products at a spatial and temporal scale not previously available. Here, we utilize ECOSTRESS temperature products to assess fluctuations in thermal habitat suitability for an endangered fish species and we also demonstrate the utility of the data set to assess water surface and bulk temperature changes following completion of two tidal wetland restoration efforts in the San Francisco Estuary and Sacramento–San Joaquin River Delta (Bay Delta). During all hours of the day throughout the 2018–2022 study period, key Delta Smelt habitat areas including Honker Bay, West Suisun Bay, and Mid Suisun Bay had greater than 75% of their areas suitable; there were several instances during which less than 25% of the area was thermally suitable generally coinciding with heatwave events. Overall, midday hours are less frequently suitable than morning and evening hours. Early evidence from ECOSTRESS indicates that there may be a decrease in surface water temperature of up to several degrees in tidal restoration areas, but more data is needed to show statistically significant outcomes. The ECOSTRESS record began in summer of 2018, and a longer record is needed to fully capture temperature changes associated with wetland processes and restoration efforts. Future applications of ECOSTRESS products can the capacity to help further understand ecosystem conditions and how restoration efforts affect water temperature, informing decisions that benefit Delta Smelt and other at‐risk aquatic species. Plain Language Summary: Water temperatures are rising in the San Francisco Estuary and Sacramento–San Joaquin River Delta (Bay Delta), negatively affecting endangered fish survivability. An instrument aboard the International Space Station called the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) can give vital information about Bay Delta water temperatures at a broader scale than field measurements with reasonable spatial resolution and on a more frequent basis than previously available to understand where habitat management efforts would be most useful and even assess the impacts of previous restoration efforts on temperature. Here, we look at how favorable the Bay Delta water temperatures are for endangered Delta Smelt and assess the effects of two completed restoration efforts on water temperature (The Tule Red and Winter Island tidal wetland restoration projects). As managers and decision makers continue to evaluate actions toward protecting endangered aquatic species, ECOSTRESS can provide information to support the decision making process. Key Points: Aquatic thermal habitat conditions vary diurnally and spatiallyMidday periods and inland regions have unsuitable habitat for the Delta Smelt most frequentlyECOsystem Spaceborne Thermal Radiometer Experiment on Space Station shows early evidence of surface water temperature cooling benefits from tidal wetland restoration projects of up to 5.4°C [ABSTRACT FROM AUTHOR]

The San Francisco Estuary and Sacramento–San Joaquin River Delta (Bay Delta) is a highly sensitive and critical habitat for the Delta Smelt, an endangered endemic fish, with water temperature being a key determinant of habitat suitability. This study investigates the relationship between open water surface and subsurface conditions from spaceborne thermal measurements (ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) and Landsat-8) and in situ sensor data from the California Data Exchange Center (CDEC) to produce estimates of spatially continuous bulk temperature in the Bay Delta. We found that ECOSTRESS and Landsat-8 surface temperature measurements are well-correlated with bulk water temperatures ($N =236$ and $r = 0.907$ and $N = 226$ and $r = 0.976$ , respectively). For the ECOSTRESS-in situ comparison, accounting for time of day improved the correlation between surface and subsurface conditions ($r = 0.946$ , 0.881, and 0.944 for morning, midday, and evening, respectively). We found that ECOSTRESS surface temperatures were warmer than bulk temperatures in the midday period (2 °C peak at 2 P.M.) and cooler in the morning and evening periods (−1°C peak at 6 A.M.). We also found that a simple harmonic regression model can capture the diurnal variability of the skin effect to predict bulk water temperature (root-mean-square error (RMSE) = 0.809°C). With ECOSTRESS, we found that across the Bay Delta, including open waters and pelagic bays, temperature conditions causing stress and mortality for the Delta Smelt were persistent throughout the day during summer months. ECOSTRESS is a unique dataset capable of informing conservation efforts in the Bay Delta. [ABSTRACT FROM AUTHOR]