Some of the industries that add to the pollution of the Yamuna include printing, electroplating, soap manufacture, food processing, rubber, plastics, chemicals, and leather tanning. Going downstream, most natural rivers become finer grained, higher in discharge, gentler in slope, and more sinuous. These changes have important effects on vegetation, flood characteristics, ecological habitats, and so forth. 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, Paola et al. (1992) produced downstream fining 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 ruled out abrasion effects. Selective deposition appeared to be a natural consequence of the transport and deposition of sufficiently poorly sorted or bimodal gravels and could account for fining rates observed in natural gravel rivers.