Experiments on downstream fining of gravel .1. Narrow-channel runs

We present the results of three laboratory experiments in which longitudinally sorted deposits were formed by feeding poorly sorted sediment at the upstream end of a narrow, 45-m-long channel. The input sediment had a median size of 6 mm and included significant amounts of material up to 64 mm and down to 0.2 mm. Water discharge was constant at 49 L/s and sediment discharge varied from 0.048 to 0.19 kg/s. Downstream fining was produced in all three runs; the variation in sediment-feed rate had relatively little effect on the fining profiles. In all three runs, the formation of a longitudinally sorted deposit was mediated by the formation of a coarse surface layer. The surface layer remained at the top of the deposit during aggradation by continually reforming itself at the deposit surface. The coarse surface layer fined by approximately a factor of 2 in D-50 and D-50 consistently in all three experiments. The deposit (subsurface) fined less, with D-50 fining more strongly than D-50. The short channel length and the relatively high rate of deposition rule out clast abrasion as the source of fining. The experiments suggest that selective deposition of the coarsest clasts due to unequal mobility is capable of producing fining rates comparable with the highest rates observed in nature.