The planning and design of a sustainable reservoir require the accurate prediction of flow and sediment deposition patterns. Numerical simulation of flow in reservoirs is important in order to determine the detailed flow pattern. This typically includes flow separation at the inlet, accompanied by recirculation and stagnation regions. The aim of the present paper is to study the influence of the reservoir geometry on the flow patterns, by means of physical and numerical modelling of the flow behaviour. Different geometries were considered (characterized by different lengths of the basin), but with fixed inflow discharge and downstream water depth. In spite of the symmetric setup, leading to symmetric hydraulic and geometrical conditions, the physical and numerical experiments generally produced an asymmetric flow pattern that can easily switch sides depending on the initial and boundary conditions. The numerical simulations revealed that the observed asymmetry in flow patterns can be explained by the high sensitivity of the flow with respect to initial and boundary conditions. These numerical simulations succeed in predicting, with a satisfactory agreement compared to experiments, the influence of the length of the basin on the flow pattern.