Downstream implications of large river diversions on thermal fish habitats in the coastal zone

Abstract

Large river diversions are becoming an increasingly prominent coastal management strategy to address land loss, water quality, and other major sustainability challenges. The U.S. Louisiana Gulf coastal region features a high degree of flow intensification due to high rates of sea level rise, increasing heavy rain events, and engineered hydromodification (e.g., urban development). These uncertain factors confound the determination of the long-range biological implications of coastal river diversions. Here we present an assessment framework to quantify downstream implications of large diversions on thermal habitats using a sub-daily soil and water assessment tool (SWAT) model coupled with a physically based Hydrologic Engineering Center-River Analysis System (HEC-RAS) model. The models successfully simulated the water temperature response to the Teche-Vermilion Project which is the first large-scale freshwater diversion in the State of Louisiana, performing well against the available measurements for stage (NSE >0.7, BIAS <6 %), discharge (NSE >0.6, BIAS < −15 %) and water temperature (NSE >0.9, BIAS <1.5 %). A robust counterfactual scenario analysis was employed to assess the impact of a variety of plausible future forcing regimes (sea level rise, tributary runoff, shading) on the diversion/downstream temperature response. The results indicated that the freshwater diversion modified downstream mean annual temperatures by 0.3–0.5 °C as far as 60 km downstream while canopy reductions yielded changes on the order of 1.0 °C. Dynamic habitat suitability metrics for three native fish species and strong direct relationships were established between annual thermal habitat suitability metrics and annual downstream temperatures. While other studies have identified the effects of coastal river diversions on sediment, nutrient, and salinity fields, the effects on downstream thermal habitats has been understudied. Water temperature is critical not only for constituent reaction kinetics and transport, but also to the reproductive success of downstream fish populations. By addressing the master variable of temperature in the representative setting of coastal Louisiana, this study addresses a significant knowledge gap on the thermal implications of diversions in similar settings worldwide.