Under uniform flow conditions, compound channel flows are characterised by an horizontal mixing layer developing at the junction between the flows in the main channel (deeper and faster) and in the floodplain (shallower and slower), e.g. Shiono & Knight (1991). Maintaining the total discharge corresponding to the uniform flow and imposing a disequilibrium in the upstream discharge distribution generates a non-uniform flow where lateral mass exchange occurs between channels along the flume. Two cases are distinguished: an excess and a deficit in the inlet floodplain discharge. The over- and under-feeding of the floodplain reduces and increases, respectively, the difference in velocity between both channels at the inlet, compared to uniform flow conditions. This paper investigates the effect of the magnitude of the upstream disequilibrium on the turbulent shear layer. The influence of lateral mass exchange on three-dimensional velocity field and on Reynolds stresses is assessed.