Evaluation of the Effect of Delta Outflow on the Water Temperature of the Low Salinity Zone and Potential Management Implications for Delta Smelt

Abstract:

The Low Salinity Zone (LSZ) is the region of the Sacramento-San Joaquin Bay-Delta with salinity at depth between 0.5 and 6 psu and is a historically important rearing habitat for several estuarine fishes. However, water temperature around the LSZ is often high enough during the summer to stress native fishes, specifically Delta Smelt, with the Delta generally warmer than the Bay during summer. This study examined whether shifting the LSZ and X2 could affect the water temperature in the LSZ and change the habitat quality for native fishes. The three-dimensional UnTRIM Bay-Delta model was used to predict salinity and water temperature throughout the Bay-Delta under a range of conditions and results were extracted for the LSZ. We used the summer of 2017 as a baseline condition because it represented a period when water temperatures in the LSZ were warmer than 20°C. The influence of LSZ location on LSZ temperature was evaluated by adjusting Delta outflow downward, by increasing exports, and decreasing Delta tributary inflows to shift X2 east by 3, 6, 10, and 15 km. Shifting X2 and the LSZ east resulted in an increase in the water temperature of the LSZ, with larger shifts resulting in greater increases in average LSZ temperature. Shifting X2 east also resulted in larger proportions of the LSZ area being warmer than 22°C. The maximum predicted daily increase in the LSZ water temperature ranged from 0.3°C (3 km shift) to 1.2°C (15 km shift). The maximum predicted monthly averaged increase in LSZ temperature ranged from 0.1°C (3 km shift) to 0.9°C (15 km shift). Shifting X2 east resulted in a general shift of the LSZ toward the east. Generally, these results indicate that shifting X2 east during a wet year would increase the average water temperature in the LSZ during the summer, potentially deteriorating Delta Smelt habitat quality and availability. However, the relatively small magnitude of temperature change and the operational feasibility of shifting Delta outflow through prescribed export ranges and reservoir release patterns suggest limited impacts to population dynamics in this regulatory construct.