The simulation of run-off in an alpine catchment area equipped with complex hydropower schemes is presented by the help of an especially developed tool, called Routing System, which can combine hydrological modelling and operation of hydraulic elements. In the hydrological forecasting tool tridimensional rainfall, temperature and evapotranspiration distributions are taken into account for simulating the dominant hydrological processes, as glacier melt, snow pack constitution and melt, soil infiltration and run-off. The advantage of this object-oriented modelling tool is the integration of routing in rivers as well as hydraulic structures such as water intakes, reservoirs, turbines, gates and valves. The model allows simulating the operating mode of complex storage hydropower plants and its impacts on the downstream river system for different scenarios, taking into account natural and anthropogenic influences. The paper presents the modelling of the complex Oberhasli hydropower scheme, probably one of the most complex one in Switzerland. The development and calibration of the hydrological model is explained. First results are shown at the example of the flood event of 2005. The effect of the existing reservoirs and their management on flood routing is highlighted.