Juvenile fish stranding induced by upstream gate operation: A risk assessment through eco-hydraulic modeling

Rapidly changing flows in regulated rivers can significantly impair freshwater biotas. At present, this issue is mostly addressed by hydropeaking research. In contrast, only limited information is available concerning the environmental effects of sharp streamflow variations determined by manmade manipulation other than hydroelectricity generation. In this paper, we assessed the stranding risk of juvenile marble trout (Salmo marmoratus), an endemic species of particular interest in the study area, during rapid streamflow decrease controlled by upstream gate operation. Our investigation focused on the falling limb of a regulated flood hydrograph in a residual-flow reach of the Ticino River, located few kilometers below its outlet from Lake Maggiore (Northern Italy). The stranding risk assessment was carried out by coupling two-dimensional unsteady flow simulation and standard habitat suitability modeling. Specifically, we adopted univariate habitat suitability curves, and a threshold for the critical rate of decrease of the water surface elevation (downramping rate), commonly used in hydropeaking assessments. Our results evidenced potential stranding occurrence of the target fish species, and possible mitigation by reducing the rate of streamflow decrease before restoring the minimum flow after flood spilling. However, more reliable estimates, including partitioning the stranding risk into classes, are challenged in our opinion by difficulties arising from extensive field validation and related selection of model parameters. In a global context characterized by the urgent need of mitigating the hydro-morphological alteration of regulated rivers, we are confident that the presented approach could support an improved streamflow management in river reaches below reservoirs and regulated lakes, therefore arousing attention and research advancement by the scientific community.