The purpose of this study is to understand the evolution process of river bed whose sediment size distribution is extremely wide. As a first stage of such study, movable bed experiments were conducted under the condition that the bed was composed of three different grain-sizes of sediment whose diameters take a different order of magnitude. One represents a group of larger size of sediment which can not move at all, and the others are in the group of medium or finer one which moves as a bed-load or a suspended load. In the process toward the stable state of bed, it was found that a typical vertical sorting occurred, and the characteristics of the bed structure were discussed on the basis of experimental results.
