Downstream changes in the lithology and grain size of fluvial gravels, the Watarase River, Japan; evidence of the role of abrasion in downstream fining

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