The Iinashi River rises from the granitic region in the Chugoku Mountains and flows northward into the Naka- Umi, an inland sea. It forms an alluvial plain in its lower reaches. The author analysed the grain size and mineral composition of the river bed materials on the alluvial plain. The relationship between the feature of the bed materials and the formation of the alluvial plain is discussed. The results are as follows.
The river bed materials are composed of granitic sands and gravels finer than-5.5 phi. The histograms showing the frequency distribution of the grain sizes on weight basis are divided into two groups: the No. 1- 2 group in the lower reaches and the No. 3- 10 group in the upper reaches. The former group shows nearly a unimodal distribution with its peak at around-1.5 phi. The latter group has a polymodal distribution with four peaks. On an average, a distinct “valley”in the distribution appears in the part of around -2.75 phi.
The granitic bed materials are divided into rock fragments, quartz grains and feldspar grains. The author examined their proportions according to grain sizes. As a result, the proportion of rock fragments decreases in the grain size ranges finer than -3.00 phi, whereas the proportion of quartz grains increases rapidly between -3.00 phi and -2.50 phi. It may safely be said that in the Iinashi River the bed materials composed of granitic rock fragments are unstable and easily destroyed in the grain size ranges between -3.00 phi -2.50 phi, but quartz grains remain selectively.
The Iinashi River alluvial Plain is divided into three landforms: valley plain, fan and delta. The inclintion of fan is smaller than any other fan in Japan. The reason is that the granitic bed materials are very easy to be destroyed. The inclination of delta is smaller than fan, because the bed sediments in the lowest reach are composed of sandy mineral grains.