We present a Particle Tracking Velocimetry system for the study of two-size mixtures of coarse spherical glass beads entrained by a shallow turbulent water flow down a steep channel with a mobile bed. The particle diameters were 4 and 6mm, the channel width 6.5mm and the channel inclination typically 12.5%. The water flow rate and the solid discharge were kept constant at the upstream entrance. They were adjusted to obtain bed load equilibrium, that is, neither bed degradation nor aggradation over sufficiently long time intervals. Flows were filmed from the side by a high-speed camera. A specific image processing algorithm was developed to determine the position, the trajectory and the state of movement (rest, rolling or saltating) of each spherical particle. Experimental investigations were made with the system. Comparison with previous results confirm that the collective motion of rolling is specific to particles of uniform size. The use of a two-size mixture resulted in a bed mainly formed by the 4mm beads. To understand the downward migration of the fine particles, an equilibrium flow with only 6mm beads and a very small rate of fine particles was also analyzed. For the migrating beads, the transitions between rest and rolling were principally caused by a neighborhood of four large beads. This is due to the loose packing of large beads favouring the downward migration of fine particles.