River dams interrupt essential migration routes of fish and other aquatic organisms. One of the most important mitigation measures that enable aquatic fauna to migrate, especially during spawning, are fishways. The paper presents all the main results of the numerical model PCFLOW2D, which proved to be a useful tool in designing a vertical slot fishway (VSF). The presented analysis includes the quantification of the effect of various numerical and geometrical parameters. Extensive field measurements of flow characteristics in the operating fishway at the hydro power plant Arto-Blanca, which are employed for the model's validation, are discussed as well. The results of simulations for various longitudinal slopes of the VSF are presented. In addition to maximum flow velocities and discharge, other important hydraulic parameters affecting fishway efficiency were analyzed, such as velocity field and distribution of turbulent kinetic energy dissipation.