Shallow depth flow fields and low velocity magnitudes are often challenges for traditional velocity measuring instruments. As such, new techniques have been developed that provide more reliable velocity measurements under these circumstances. In the present study, the two-dimensional (2D) surface velocity field of shallow basins is assessed by means of Large-Scale Particle Image Velocimetry (LSPIV). The measurements are carried out at the water surface, which means that a laser light sheet is not needed. Depending on the time scales of the flow and the camera characteristics, it is even possible to work with a constant light source. An experimental application of this method is presented to analyze the effects of shallow basin geometry on flow characteristics in reservoirs where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics. The flow and boundary conditions that give rise to asymmetric flow are presented. Asymmetric flow structures were observed starting from basin shape ratios that are less than or equal to 0.96. By decreasing the basin length and increasing the shape ratio to greater than 0.96, the flow structure generally tends towards a symmetric pattern.