Doubts have been expressed about the ability of either abrasion or sorting to explain strong downstream fining of river gravels. We describe pronounced fining over a short distance in a Scottish river that has no human disturbance or lateral input of water and sediment. Measured abrasion rates are far too small to explain observed downstream fining, but bed-load trap measurements and the dispersion of magnetic tracer pebbles in six subreaches both show sorting during transport. The observed downstream fining is also simulated well by a numerical sediment routing model using hydraulic and transport laws consistent with our field measurements. The geomorphological cause of the strong fining is slope reduction above a local base-level control. In this common situation the development of downstream fining is part of the river's tendency to minimize downstream variation in bed-load transport rates, and can proceed far more rapidly than the major aggradation otherwise required for equilibration.