INCISED MEANDERS IN THE CHUGOKU MOUNTAINS, SOUTHWEST JAPAN

Abstract

In the Chugoku Mountains, Southwest Honshu, there are many incised meanders, but few free meanders. The purpose of this paper is to inquire into the distribution and the origin of incised meanders in the Chugoku Mountains, and to consider the relationship between their wave-lengths and drainage areas and also their relation to rock properties. As is shown in Fig. 1, the distribution of the incised meanders in the Chugoku Mountains coincides with the distribution of planation surfaces which are presented as “gentle slopes” in the 1:500, 000 geomorphological map (Economic Planning Agency, 1968). A half of all the incised meanders cuts into the Kibi Plateau Surface, and they are concentrated in the areas of the Kibi and Iwami Plateaus. While one third of all the meanders cut conse-quently the boundary slope between the Kibi and the Backbone Ridge Surface. The in-cised meanders which cut into the erosion surfaces are either entrenched meanders and ingrown intrenched meanders. They may be inherited from free meanders. The incised meanders that develop on the boundary between erosion surfaces are of the type of ingrown meanders, and have no erosion surface that might have produced free meanders, because they can develop during an initial cycle of erosion. In comparison with incised meanders in Paleozoic rocks, those in granitic rocks are greater in the width of the meander belt and of the valley. According to the histogram of sinuosity of incised meanders (Fig. 5), incised meanders in granitic rocks have greater sinuosity and variance, the mean sinuosity being 2.10 and the variance 0.75, than those in Paleozoic rocks, of which the corresponding values are 1.96 and 0.59 respectively. Many incised meanders in granitic rocks are structurally controlled by joints (Fig. 4), and have an irregular and zigzag pattern of stream courses. On the contrary, those in Paleozoic rocks generally form regular and smooth curve. Although geologic structure and lithology affect the shape and scale of the incised meanders, the incised meanders in the Chugoku Mountains develop in all sorts of bedrocks, and have no preference to particular rocks. Figure 6 shows the relation between the wave-length and the drainage area for the incised and free meanders in the Chugoku Mountains as well as for those in Europe and North America. According to this, there is no substantial difference between the relations for the Chugoku Mountains and for. Europe and North America. However, the. streams in the Chugoku Mountains have not manifestly underfitted due to climatic change, as in the case of Europe and North America. From the above mentioned relations, the former drainage area before stream capture can be estimated for three rivers, as shown in Fig. 6.