ON THE LONGITUDINAL PROFILES OF THE ASAKE, MITAKI AND UTSUBE RIVERS, MIE PREFECTURE

Abstract

The Asake, Mitaki and Utsube Rivers rise in the Suzuka Mountains, run eastward through the alluvial plain and flow into the Ise Bay (Fig. 1). The geological conditions of the river basin in the mountainous areas present striking contrast each other. The Asake River basin, for instance, consists only of granite, while that of the Mitaki River contains Paleozoic rocks in addition to granite. And most portion of the Utsube River basin is composed of Paleozoic rocks. Meanwhile, the longitudinal profiles of the three rivers have a common characteristics : they have concave reaches in the upper stream and straight reaches in the lower. Among the upper stream, the Asake River has the greatest concavity and the Utsube River the smallest (Fig. 2, A. B). The purpose of this study is to investigate the cause that has created the difference in the concavity in the longitudinal profiles of the three rivers. Thus the observation is made not only on the particle size of the bed material but also on the grain lithology. The study of the relationship between the particle size of the bed material and the change of channel slope downstream as each rivers has led to the following findings : (1) The distribution of maximum size grains in the bed material of each river changes exponentially (Sternberg's law). However, it is subject to the grain lithology and is not corre-lated with the change of channel slope (Fig. 2, C). (2) The downstream change of channel slope-concavity-is closely correlated with the change in median size of bed material downstream (Fig. 2, B. D). (3) L, ithological separation of bed material, however, indicates that the change in the median size of bed material downstream does not correlated with the channel slope. The change in the size of bed material is characterized by the peculiarity in the distribution of each kind of rock involved. For instance, granite breaks up rapidly into mineral particles, 2-3mm in diameter, but the diminishing rate of the particle size, further on, is very small. The di-minishing rate of particle size of Paleozoic rocks is very small as compared with that of granite (Fig. 2, D). (4) Therefore, the downstream change of bed material size is influenced by its lithologic composition. And the lithological composition of bed material exactly reflects the geological conditions of each river basin in the mountainous area (Fig. 3). Reflecting the lithology of the mountainous part, the bed material of the Asake River is composed of granite, and the downstream change of the bed material size indicates that of granite debris, so the Asake River produces great concave profile in its upper reaches and the straight profile in its lower reaches. On the other hand, the Utsube River, mainly consists of Paleozoic rocks, has the bed material of Paleozoic rocks, so it produces small concave profile. The longitudinal profile of the Mitaki River has intermediate character of two other rivers, due to the lithology of her basin. Concludingly, it may be said, that the difference of the concavity of longitudinal profiles of these rivers are mainly due to the difference in the lithologic conditions of the mountainous part of each river basin. The more quickly the particle size of bed material diminishing down-stream, the more the longitudinal profile becomes concave. Then, it may be said, that the concavity of longitudinal profiles of alluvial rivers is one of good examples of the configura-tions formed under the influence of lithologic condition (ROCK CONTROLLED TOPOGRAPHY).