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.