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THE bed surface of most gravel rivers is considerably coarser than the sub-surface or the gravel load transported over it, a phenomenon affecting river dynamics, morphology and ecology. The coarse surface layer, often called an armour or pavement, has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes^1,2; and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile^3,4. Where bedload supply is cut off below dams, selective erosion causes coarsening^5,6. Using a simple quantitative model, we propose that surface coarsening develops in gravel-bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load. We present laboratory results which support this hypothesis, and show that supply reduction causes changes in bedforms and progressive confinement …