Research on lateral exchange of streamwise momentum between parallel flows in open channels has mainly been focused on compound channels composed of a main channel and a floodplain, on mixing layer development downstream of river confluences, and on partially vegetated channels. This study aims to establish the mechanisms responsible for streamwise momentum exchange between concurrent parallel flows subject to different bed roughnesses. The contribution of momentum exchange of each mechanism is determined on the basis of flume experiments. For an initially uniform flow that experiences a bed with two parallel lanes of different roughnesses, three mechanisms for exchange of streamwise momentum can be distinguished: cross-channel secondary circulations, turbulent mixing resulting from vortices acting in the horizontal plane, and mass transfer from the decelerating flow over the rough-bottomed lane to the accelerating flow in the parallel smooth-bottomed lane. The mass transfer and associated momentum transfer across the channel cause a gain in longitudinal momentum. The secondary circulations are driven by turbulence anisotropy and feature a main cell that extends over the full water depth, which is centered at the smooth side of the smooth-to-rough transition. The gain of momentum corresponding to the mass transfer in the developing reach is on the same order of magnitude as the momentum exchange by turbulent mixing and of that by secondary circulations in the most downstream position, where the flow is nearly developed. The contribution of the secondary circulations to the exchange of streamwise momentum between the parallel flows gradually becomes dominant over the contribution of turbulent mixing when depth increases.