Abstract: In order to fulfill the requirements of the EU Water Framework Directive concerning the restoration of fish passage in rivers, various types of (near-natural and technical) fishways have been built in recent years. For optimization of planned structures as well as for monitoring of the performance of existing fishways, their hydraulics has to be studied. 3D hydrodynamic numerical modelling is a useful tool to predict hydraulic quantities and analyse flow pattern. Many studies using hydraulic simulations deal with technical fishways. In contrast, nature-oriented fishways have rarely been numerically modelled so far. In the framework of R&D activities at Karlsruhe Institute of Technology, a high resolution 3D-hydraulic model of a section of a nature-oriented bypass channel (rock cascade pass) has been developed. The basis of a hydraulic model is a 3D-model of the fishway’s topography. Since fishways of a nature-oriented design type are characterized by highly irregular structures, the build-up of such a model is very challenging. In this study, the method of terrestrial laser scanning was used for detailed geodata acquisition. The hydraulic model was validated using, inter alia, acoustic Doppler velocity measurements. Simulations using the computational fluid dynamics software FLOW-3D were performed for two hydraulic scenarios. The simulated hydraulic quantities (velocities, water levels) and their distribution along the vertical and horizontal axes have been investigated and compared to critical values concerning fish passability specified in technical standards. A method of categorization of flow patterns is proposed, which enables a schematic visualization of complex flow conditions. In this way, potentially critical zones can be located and specific flow patterns can be identified. This method of analysis has a high potential in terms of addressing environmental issues concerning nature-oriented fishways and river restoration projects.