Field evidence for rapid downstream fining of river gravels through selective transport

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 river9s tendency to minimize downstream variation in bed-load transport rates, and can proceed far more rapidly than the major aggradation otherwise required for equilibration.