This paper presents results from an experimental investigation of rough turbulent open channel flow over transverse square bars. Bar spacing was systematically varied between 2 and 20 bar heights. Velocities were measured by means of Particle Image Velocimetry (PIV), which captured details of flow fields above and between the bars. The streamlines calculated from the time-averaged flow velocities show the shape of persistent vortices in the gap between the bars. Further analysis is based on Double-Averaged Navier-Stokes equations (DANS). Momentum exchange between the flows above and within the gaps is explored using spatial quadrant analysis and the decomposition of the spatially averaged turbulent stress.