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In order to relate depositional landforms with the grain size distributions present on them in the Kiso and Yahagi River alluvial plains and the shallow marine delta of northwestern Mikawa Bay, the sediments were analyzed by the methods of mechanical sieving and photoextinction-sedimentation. Correlation diagrams between the means and skewnesses of the alluvial plain sediments showed that plots for sandy deposits could be clearly distinguished from those for fine, muddy sediments, and that there was a strong negative correlation between mean and skewness. From cumulative grain size distribution curves plotted on normal probability paper, the author quantitatively separated some component populations showing normal distributions. The resultant distributions of the means and standard deviations of the separated component populations indicated all the muddy sediments to be composed of two to four component populations which could be divided broadly into sand and clay populations. In most of the sediments, the means and standard deviations for the clay populations showed a fairly constant mean value with poor sorting, which may be a result of the weathering processes.　0n the other hand, those for the sand populations exhibited various means with better sorting, which may have resulted in a complex manner from mechanical attrition of rock-composing mineral grains and from the hydraulic conditions. Correlation diagrams between the mixing proportions of the sand populations and the means, and between these and skewness showed strong negative and positive correlations, respectively. This indicates that the statistical parameters were mostly determined by the mixing proportions of each component population. As the mixing proportions are thought to be related to the actual hydraulic conditions concerned, the sediments should contain high proportions of sand populations under a great flow intensity. Thus, the surface sediments of steeply sloping alluvial fan and of natural levee near a river channel had dominant sand populations, whereas the sediments of the backswamp contained high proportions of clay. Based on histograms showing the average relative frequency distribution of the sediments in each geomorphic element, it is clear that all muddy sediments had two peaks in common, in the 1.5 - 3.5 phi and the 6－8 phi range, with a “valley” at around 5 phi, throughout all geomorphic elements. The coarser peaks clearly correspond to the sand populations and the finer ones to the clay populations, and the “valley” at around 5 phi represents a relative “deficiency” of grains. The geomorphological features and grain size characteristics of the Yahagi River alluvial plain showed some striking peculiarities, when compared with those of the Kiso River. These peculiarities are attributable to the geology of the drainage basin, which is composed mostly of granitic rocks.