Sediment transport is essential to the source-sink systems; however, the interaction between two complex multiscale nonlinear systems, turbulence of the river flow and wide size sediment, has heretofore restricted our understanding of sediment motion. We have conducted flume experiments deploying a video-based technique that records sediment transport rate of each particle size at 1 s resolution. The observations reveal detailed interactions between flow and particles of sizes ranging from 0.5 to 32 mm, such that small suspended particles (< ~ 5 mm) keep swirling in the wake vortices of the keystones (larger than 20 mm) until large to very-large-scale coherent structures destroy the wake vortices and bring the small particles downstream. Keystones destabilize consequently as the surrounding small and intermediate particles move, and in turn, a group of sheltered particles is entrained following the dislodging of the keystones. This heuristic model highlights the interactions of turbulence and different-sized particles.