The author has reported, recently in this journal (Moriyama, 1977), a study on the grain size distributions of surface sediments of the Kiso River alluvial plain which exemplyfied Japanese alluvial plains. In this report, standing on the similar viewpoint to the previous report, he firstly clarifies the relationship between landforms and grain size distributions of the Yahagi River alluvial plain sediments, and then give final explanation of all analyzed results by him (Moriyama, 1976b, 1977). Fig. 1 shows the landforms of the Yahagi River alluvial plain and a distribution of sampling points of sediments. Table 1 lists the statistical parameters (calculated by moment method), of all samples and Fig. 2 illustrates a correlation diagram between the mean and skewness of the statistical parameters. In this figure the plots of river sands are clearly distinguished from those of flood plain sediments; however each muddy sediment can not be distinguished individually, i.e., YN and YB overlap each other. A strong negative correlation between the mean and skewness can be seen in this muddy sediments as for those of the Kiso River alluvial plain. In order to consider precise sedimentological meanings of the muddy sediments, firstly the author drew the cumulative grain size distribution curves on normal probability graph paper for all samples (Figs. 3_??_5). Then, he segregated quantitatively each curve into several component populations, each having the normal distribution and calculated the mean, standard deviation and mixing proportion for each component population employing the method developed by Inokuchi & Mezaki (1974). Distributions of mean, standard deviation and mixing proportion for each component population are shown in Fig. 6. All samples are composed of two to four component populations and can be grouped into sand-gravel and clay populations, lacking component populations in silt size. The sand populations are well sorted while the clay populations are of relatively poor sorting. Using the averaged frequency of all size classes for each geomorphic element, histograms and relative frequency distribution curves were constructed as shown in Fig. 7, in which also included were the analyzed samples for the Kiso River alluvial plain sediments and deltaic shallow marine bottom sediments in the northwestern Mikawa Bay. Generally, the shapes of frequency distributions of all geomorphic elements except for sandy deposits show common features of bimodal distributions and reveal a “valley” around 5 phi. Comparing the shapes of histograms of the Kiso River alluvial plain sediments with those of the Yahagi, we can recognize a common “shoulder” at 0_??_2 phi in YN and YB, and even in SMp, while IF, KN and KB have no “shoulder” at O_??_2 2 phi. This implies that the size characteristics of the sand population can be attributed to that of the original materials from which sediments are supplied to the alluvial plain. Larger grain size of the Yahagi River alluvial plain sediments are thought to be caused by the coarser-grained granitic rocks of the drainage basin. Relative frequencies of the sand population decrease in the order of, IF>KN>KB, and YN>YB. The author's conclusive explanations can be summarized as follows; As clay populations show fairly consistent characteristics in all geomorphic elements, it might be produced originally by weathering processes active in the weathered mantle of mountain and hill slops, not by hydraulic conditions. On the other hand, the size characteristics of sand population are basically affected by the grain size distribution of rock-forming minerals in the drainage basin rather than by hydraulic conditions at flood deposition. The mixing proportions of sand population, however, are considered to be determined by the intensity of flood flows.