## 2 Materials and Methods  
### Case Study Site  
The Austrian hydropower plant (HPP) Kirchbichl on the River Inn was built between 1938 and 1941 as a diversion plant, however, it is operated as a run-of-river power plant [28]. At Kirchbichl an omega-shaped loop of the River Inn, with a length of 3.5 km, is cut off by a \(\sim\)1 km long headrace channel (Figure 1). In order to increase the fall head of the HPP, the River Inn is impounded to 6 m by a weir at the beginning of the river loop. The HPP utilizes the head difference across the river loop, which varies between 7.5 and 9.7 m, depending on the discharge. Three turbines with a design discharge of 250 m\({}^{3}\) s\({}^{-1}\) and an output of 19.2 MW generate an average of 130.6 GWh a\({}^{-1}\) of electricity. Currently, an extension project is under construction. The extension project was developed on the basis of three complementary goals: (1) To achieve compliance with the EU Water Framework Directive by improving the aquatic ecosystem. This includes the establishment of a minimum residual flow as well as the mitigation of surge (related to upstream hydropower plants) and the establishment of fish passage. (2) Compliance with the EU Floods Directive to improve flood protection at the existing HPP. (3) Following the EU Renewable Energy Directive, the extension project includes a refurbishment as well as an increase of the design flow of the existing HPP. In the future, the extended HPP will have an increased design discharge of 484 m\({}^{3}\) s\({}^{-1}\) (plus additional 15 m\({}^{3}\) s\({}^{-1}\) at a HPP for residual flow at the weir). The new total output of \(\sim\)38 MW will generate about 165 GWh a\({}^{-1}\) of electricity. The catchment size of the gauging station Bichlwang is 9310 km\({}^{2}\) and an analysis of the time series data from 1985 to 2011 established a mean discharge (MQ) of 293 m\({}^{3}\) s\({}^{-1}\), a maximal mean discharge (HJMQ) of 374 m\({}^{3}\) s\({}^{-1}\) (1999), a mean flood event (MJHQ) of 1187 m\({}^{3}\) s\({}^{-1}\) and a maximal discharge (HQ) of 2454 m\({}^{3}\) s\({}^{-1}\) (23.08.2015), with typical discharges of \(\geq\)400 m\({}^{3}\) s\({}^{-1}\) in the summer months (May-August). The fish region in this section of the River Inn is assigned to \"Epipotamal large\" (barbel region); however, due to the presence of glaciers in the catchment, the maximum summer water temperatures are \(\sim\)15 \({}^{\circ}\)C. These characteristics are indicative of an alpine river with a strong glacial influence, which is also reflected in the composition of the fish fauna as well as their biomass. A mean fish biomass of 25.1 kg ha\({}^{-1}\) (296 fish ha\({}^{-1}\)) in the Kirchbichl reservoir and a mean fish biomass of 22.6 kg ha\({}^{-1}\) (134 fish ha\({}^{-1}\)) in the Langkampfen reservoir, located below HPP Kirchhoff, was assessed via a hydroacoustic survey in 2012 [29]. In addition, an electrofishing campaign was carried out in the Kirchbichl reservoir in 2012: with 461 individuals ha\({}^{-1}\) accounted for a biomass of 23.4 kg ha\({}^{-1}\). In total, only five fish species were detected, and their abundance (brown trout 6.3%, bullhead 6.7%, rainbow trout 77.9%, grayling 2.6% and common nose 6.5%; in % of the total density) indicate that the water body is affected by stocking activities [30]. Within the Austrian national monitoring framework (GZUV), electrofishing was carried out the river stretch below HPP Langkampfen in November 2008. A total of 14 fish species were found, which represents a wide range for an Alpine catchment. However, the fish density and biomass remained low at 92.1 individuals ha\({}^{-1}\) and 5.1 kg ha\({}^{-1}\). Based on the aggregation of these findings, the site-specific conditions regarding fish downstream migration can be summarized as follows: (1) The target species are potamodromous fish. (2) Due to the alpine catchment, the number of species is low and also fish biomass is comparatively low [23].