The paper presents results of experiments conducted in a laboratory flume with a compound cross-section consisting of a central main channel (MC) and two adjacent floodplains (FP). The focus of the study has been on turbulent mixing layers and large-scale coherent structures that form at the interfaces between the MC and two FP at non-uniform flow conditions in the streamwise direction. In the experiments, the streamwise non-uniformity is generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance, keeping the total flow rate (Q = 114 L/s) constant. The experiments started with initially uniform flows (8 L/s in each FP and 98 L/s in MC), which then have been modified by redistribution of the water discharge between MC and FPs. Three unbalanced inflow conditions have been tested, with a FP inflow of 4, 12 and 16 L/s at each FP. The discharge imbalance results in a transverse depth-averaged mean flow (termed herein ‘transverse flow’) from MC to FP or vice versa, quantified by the depth-averaged transverse velocity Uyd. Based on ADV measurements with 1 or 2 side-looking probes, it is shown that transverse currents towards the MC with a velocity ratio Uyd / Uxd higher than 5% can entirely displace the mixing layer, shear-layer turbulence, and coherent structures, shifting them into the MC. It is also found that streamwise uniformity is reached faster with a transverse flow towards the FP than with a transverse flow in the opposite direction.