Selective sorting and abrasion of river gravel. II: Applications

Most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers. It has been surmised that this downstream fining is due to some combination of selective sorting, by which finer grains are preferentially transported downstream; and abrasion, by which individual particles are reduced in size. Here the framework for the simultaneous treatment of both phenomena, developed in a companion paper, is used to analyze several cases of interest. In particular, wavelike aggradational profiles of permanent form are considered. An application to the Red Deer River, in Alberta, Canada, as well as several hypothetical cases, suggests the following result. In the case of quartzite, selective sorting controls downstream fining; in the case of limestone, abrasion and selective sorting are of roughly equal importance; and in the case of a mixture of quartz and limestone, abrasion ceases to be important beyond some characteristic length scale required to grind the gravel-sized limestone out of existence.