The writers studied on the geomorphology of fluvial plain distributed along the Shonai and Yahagi rivers running close each other. Geomorphological land classification maps of two fluvial plains revealed the distinct regional differences of fluvial morphology.<br> The Shonai river plain is composed of small scale natural levee, upper delta, lower delta and natural levee of the Kiso river, and the other hand, the Yahagi river plain is of fan like natural levee, fan-like upper delta and fan-like lower delta. The above mentioned re gional differences should be explained by the following data:<br> (a) Morphometric data of the Yahagi river basin, i.e., altimetric frequency, dispersion of altitude and average of relief energy, are larger than those of the Shonai. Easily weathered coarse grained granite is widely distributed in the Yahagi river basin but narrowly in the Shonai. Therefore, larger amount of detritus must be supplied to the river bed of the Yahagi as compared with the Shonai.<br> (b) Longitudinal gradient of the Shonai is gentle, only 5.3/1, 000, and that of the Yahagi is slightly steeper, 9.2/1, 000. Moreover, three gorges and three basins are distributed along the course of the Shonai ; they are, from upper to lower reaches, the Mizunami basin, the Wago gorge, the Toki basin, the Toki gorge, the Tajimi basin and the Tamano gorge. Three knick points are found corresponding to the above three gorges. This means that the sediments supplied from the mountains to the plain through these three knick points is small in volume. On the other hand, there is only one small basin along the Yahagi river, i, e., the Toyota basin. The length of the Toyota gorge, between the basin and the plain, is short enough that a lot of detritus from the granitic basement area can be easily transported to the plain through the gorge.<br> (c) The thickness of fluvial layer on the marine deposits accumulated during the Jomon transgression (6, 000-4, 000 years B. P.) in the Shonai river plain is thinner than that in the Yahagi.<br> (d) The width of the flood plain is narrow in the upper part of the Shonai river plain because terraces are well developed on both sides of the plain. The flood plain in the middle part becomes wider due to the fact that this part of the plain is surrounded by natural levees formed by a distributary of the Kiso on the right side and terraces on the left. The flood plain in the lower part is widest, for there is no topographical limitation controlling the plain width. In respect to the Yahagi river plain, the width of the flood plain is nearly same from upper to lower cousres.<br> The change of the Shonai river course has rarely occurred in the upper and middle parts of the plain, but frequently in the lower part. Therefore, the flood types of the Shonai river plain are “Concentration type” in the upper part and “Overflow type” in the middle and lower parts. On the other hand, the change of the Yahagi river course has frequently occurred. The flood water causes short cuts at the meandering section in the upper part, and overflowing from the mainstream to the adjacent lowlands occurs in the lower part of the plain.