Two processes have been considered to explain downstream grain-size reduction in gravel-bed rivers: hydraulic sorting, by which finer grains are transported faster and farther than coarse grains, and abrasion, by which individual particles are diminished in size. Most studies on the cause of downstream fining in rivers emphasize hydraulic sorting as the dominant fining mechanism. However, broken boulders are often observed in the bed material of Japanese Rivers. Also, grain-size changes may be correlated with the difference in resistance to breakdown among lithologies. The purpose of this study is to use the lithologic composition of bed material on the Watarase River to demonstrate that some downstream fining cannot be explained by hydraulic sorting alone. The study reach on the Watarase River flows on a dissected alluvial fan, which means that the river is no longer aggrading. The study reach is influenced only slightly by input of gravel from tributaries. Bed gravel in the study reach consists of several lithologies: andesite, quartz porphyry, sandstone, hornfels, and chert. The grain-size distribution and lithology of the river-bed material were examined at nine sites. There are two principal results. (1) Size distribution of the gravel is strongly related to lithology. Andesite boulders or large cobbles make up the framework sizes in the upstream part of the study reach, while chert pebbles make up the framework sizes in the downstream part. There are few andesite pebbles or chert boulders in the river bed. (2) Longitudinal changes in the composition of each size class show selective transportation by lithology in every gravel size. Because the mobility of gravel depends mainly on its size, hydraulic sorting by lithology does not occur within the same size class. These results indicate clearly that particle abrasion occurs in the Watarase River and is responsible, at least in part, for the downstream decrease in particle size of bed material.

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