There has long been debate about the relative importance of abrasion versus selective deposition of the coarsest clasts in causing downstream fining of sediment in river systems. Although high fining rates observed in many natural rivers seem to require strong selective-deposition, the ability of selective deposition to produce downstream size sorting has never been measured under controlled conditions. In an experiment using a long flume and a poorly sorted, bimodal gravel feed, downstream fining was produced by a factor of 1.3 in median size and 1.8 in 90th percentile size, over a distance of 21 meters. The experimental conditions rule out abrasion effects. Selective deposition appears to be a natural consequence of the transport and deposition of sufficiently poorly sorted or bimodal gravels and appears to be capable of accounting for fining rates observed in natural gravel rivers.