The morphodynamic changes in a confluence zone due to local tributary widening were experimentally investigated in a set-up representative of regulated piedmont confluences. A reference configuration with a constant tributary width of 0.15 m is compared to a configuration where the tributary is widened to 0.30 m over its last 0.45 m. The main channel is 0.5 m wide and the confluence angle is 90°. Comparison is made for a discharge ratio Qt/Qm = 0.11. Tests were run until equilibrium with a constant solid discharge of poorly-sorted material from the tributary. The paper presents detailed data on the morphology and on the three-dimensional flow velocity field at the confluence zone. Results revealed that the local widening of the tributary creates a pronounced heterogeneity in the flow velocities and flow depths, without having any adverse effects on flood safety in the confluence zone. Although the local tributary widening allows a reduction in the confluence angle, it locally amplifies the hydro-morpho-sedimentary processes in the confluence zone.