This paper investigates the morphologic impacts of the widening of tributaries in confluence zones by means of systematic hydraulic tests performed in a confluent laboratory flume. The confluent channels are connected with an angle of 90° and the main channel is 0.50 m wide. The tributary is 0.15 m wide in the reference configuration and enlarged to 0.30 m over a length of 0.45 m in the widened configuration. Experiments are carried out under mobile bed conditions. Sediments with a wide grain size distribution are fed in the tributary and all tests are run until equilibrium conditions. The influence of the local tributary widening is highly dependent on the momentum flux ratio between the upstream flows. The widened zone is characterized by a higher variability of the flow depth, the flow velocities and the particle size distribution, showing that tributary widening can improve the ecological value of confluence zones.