Seasonally variable relationships between surface water temperature and inflow in the upper San Francisco Estuary

Abstract

Water temperature and inflow are key environmental drivers in aquatic systems that are linked through a causal web of factors including climate, weather, water management, and their downstream linkages. However, we do not yet fully understand the relationship between inflow and water temperature, especially in complex managed systems such as estuaries. The San Francisco Estuary is the center of a critical water supply infrastructure and home to a deteriorating ecosystem with several declining fish species at the warm edge of their thermal range. We used generalized additive modeling of long‐term monitoring data to evaluate the relationship between inflow and water temperature along with its spatio‐seasonal variability. Most commonly, we found a negative temperature‐inflow relationship in which water temperatures increased as inflow decreased, up to 2°C from high‐ to low‐inflow years. However, the opposite (positive) relationship was observed in the winter months, and in the western (downstream) regions from July–September, up to − 1.2°C from high‐ to low‐inflow years. These results were upheld by models that included the long‐term temperature trend or used salinity as a proxy for location. Upstream factors likely played the biggest role in the summer when local precipitation is negligible, whereas local precipitation and the related weather conditions may drive much of the winter pattern. Although further mechanistic studies are needed to infer the direct effect of dam releases on water temperatures, these results provide a broader understanding of the impacts of flood and drought dynamics for those tasked with managing estuarine ecosystems.

Warming in the upper San Francisco Estuary: Patterns of water temperature change from five decades of data

“…However, Sacramento basin inflow has only changed (decreased) significantly in October and November for the years in our study (Hutton et al 2017). In October, inflow has a strong negative relationship with water temperature throughout our study region (Bashevkin and Mahardja 2022). The relationship is negative in November as well, but it does not extend into the northern regions of the Delta (Bashevkin and Mahardja 2022).…”

“…In October, inflow has a strong negative relationship with water temperature throughout our study region (Bashevkin and Mahardja 2022). The relationship is negative in November as well, but it does not extend into the northern regions of the Delta (Bashevkin and Mahardja 2022). Thus, the influence of river inflow on temperature trends are unclear and could benefit from further detailed study.…”

Discharge‐Mediated Temperature Management in a Large, Regulated River, With Implications for Management of Endangered Fish

“…However, no current efforts exist to manage water temperatures in the remaining lower 350 km of non-tidal river. This is likely in part due to the uncertainty in whether or not water temperatures can be affected by discharge regulation in the lower river and estuary (Bashevkin & Mahardja, 2022).…”

Evaluating Habitat Suitability and Tidal Wetland Restoration Actions With ECOSTRESS

“…Water temperature is increasing in the Bay Delta (Bashevkin et al., 2021; Halverson et al., 2021; Wagner et al., 2011), threatening biodiversity, survivability, and making conditions more stressful for endangered fish. Fish species require certain temperature ranges that support optimal functioning, with temperatures outside of that range causing stress or mortality (Davis et al., 2019b); thus, they are at risk from increasing temperatures (Bashevkin & Mahardja, 2022; Brown et al., 2016b; Davis et al., 2019a). Exposure to temperatures above their optimal temperature threshold for longer periods of time has been shown to induce negative individual and group behaviors in fish species, including alterations in predator‐prey dynamics (Davis et al., 2019b).…”