Identifying the controls over downstream fining of river gravels

A wide range of rates of downstream fining in gravel-bed rivers has been re ported. As a consequence, explanations of the phenomenon range from those t hat consider only abrasion to others that consider only selective sediment transport. This apparent confusion results in part from inconsistent defini tions of downstream fining. Analysis of results from a small gravel-bed str eam is used to show how the method of sediment sampling and the percentile used to characterize fining rates can affect the results. When fining is ad equately defined, a theoretical relationship between fining rate and draina ge-basin area is derived from established alluvial-fan relations. This rela tionship is remarkably consistent with a range of studies by numerous autho rs using often different methods. Such results suggest that fining is achie ved through the mechanism of selective transport but that the overall contr ol on fining patterns is the volume of sediment supplied from within the dr ainage basin. As such, the rate of size decline in sedimentary deposits suc h as river terraces has the potential to be used to infer past changes in c limate, tectonics, and/or relative base level. Models of this are presented , and these are shown to be broadly consistent with observations made in an ancient sandstone body. Full use of changing rates of fining as paleo-envi ronmental indicators requires further work on the rate of adjustment of fin ing profiles with respect to the rates of change in the forcing environment al conditions.