Recently it has become clear from a variety of field observations that downstream size decrease in aggrading river systems is caused in large part by selective deposition of coarse clasts. However, sorting by selective deposition has never been studied systematically under controlled conditions. The absence of a set of reference data on downstream sorting is even more striking in light of the number of theories that have been advanced recently to explain it. The main goal of the proposed project is to carry out a series of basic experiments on selective transport and deposition in poorly sorted gravelly sediments. Most of the experiments will be conducted in a large channel at the St. Anthony Falls Hydraulic Laboratory. There we will produce deposits from a poorly sorted sand-gravel mix over a distance of about 50 m. The main variables will be rate of sediment feed (and hence deposition), and channel form (straight, weakly sinuous, braided). In addition we will make detailed measurements of sediment transport and surface texture in a smaller flume at Johns Hopkins University. These data will be used to develop surface-based transport formulas that will be used in modeling the sorting observed in the large flume.
