地域素材を活用した地学の学習（８） : 芦田川の河床礫と中国地方南部の地形特性

The significance of conservation of geodiversity in global environmental conservation has found attention in Japan in recent years. The water and rock cycles, as a part of the supracrustal cycle of materials, have a crucial role in the formation and conservation of geodiversity. The 5th grade science curriculum includes a description of an ideal model on the function of running water in rivers, where it is theorized that gravel in the river become smaller and rounder as they travel farther from the river head. However, the situation of many rivers in this respect do not fit well into the ideal model. Therefore, in the light of the need to acquaint students with actual conditions in rivers, it is necessary to collect actual data on the effect of running river water on gravel, for inclusion in these curriculums. In this study, we determined the compositions, grain sizes, and shapes of river gravels at seven localities along the Ashidagawa River in the eastern part of Hiroshima Prefecture. We explored the relationship between the features of the gravels and the geology and geomorphology of the river basin. The results indicate the following: 1) Channel morphology can affect gravel composition; 2) the size and roundness of gravel varies as a function of channel slope; and 3) it is necessary to consider that huge boulders are transported by running water only with difficulty and are, thus, rounded on site by sediment particles suspended in the running water. The topography of the southern part of the Chugoku region, through which the Ashida-gawa River runs, is characterized by a stepped terrain, unlike that in an ideal model. Therefore, the features of the gravel in the Ashida-gawa River are typical examples of those of bed sediments of a river running through a stepped terrain.