THE GEOMORPHIC DEVELOPMENT OF THE MIDDLE COURSE OF THE AGANO RIVER

Abstract

Originating from the backbone range of Northeast Japan, the middle course of the Agano River flows east to west and pours into the Sea of Japan near Niigata City. The studied area is characterized by N_??_S trending tectonic structures and covered with thick tCenozonic sediments. The area is geomorphologically divided into two regions; one is the region where the river cuts a deep valley or gorge and fluvial terraces are well developed along the course of the river, and the other is the Aizu Basin occupied with well developed floodplains (Fig. 1). The river terraces are classified into eight levels along the middle course of the river. They are named as the Nishidaira, Shibasaki, Tokusawa, Nozawa, Numazawa, Mitsuai, and Yamato I and II terraces respectively. The floodplains in the western part of the Aizu Basin are classified into six levels and correlated with the Nozawa, Bange, Numa2awa, Mitsuai, Yamato terraces, and present floodplain (Fig. 2). The Tadami River had directly flowed into the Aizu Basin during the middle Pleistocene time when the Palaeo-Aizu Basin had extended beyond the hills now separating the Aizu Basin from the Tadami River valley (Fig. 8-A). After the middle Pleistocene, central part of the active lineation in the western margin of the Aizu Basin have been formed. Uplifts along this active lineation have constructed the bordering hills locating between the present Tadami River valley and the Aizu Basin. The bordering hills were finally separated from the Paleo-Aizu Basin at the time when Nishidaira terrace was formed. Therefore, the present antecedent course of the Agano River also began to cut across the bordering hills as an entrenched meandering course (Fig. 8-B). The aggradational condition was recognized in the two regions during the formation of Nozawa. terrace (Fig. 8-C). The river deepened its floor by 10.20 meters in the gorge region, and. a few meters in the Aizu Basin during the period from the formation of Nozawa terrace (ca. 21, 000_??_22, 000 y. B. P.) to the flowing down of the Numazawa pumice flow deposits (Fig. 8-D). Thick pumice flow deposits originating from Numazawa Volcano buriedd the Tadami River valley and scattered at the southwestern part of the Aizu Basin about 5, 000 years ago. Therefore, a large propotion of Tokusawa, Nozawa, and Bange terraces were completely buried under the pumice flow deposits (Fig. 8-E). As the pumice flow deposits were rapidly cut down, Yamato terrace group was formed according to local. base levels of erosion. The sediments of 150_??_200 meters in thickness have been accumlated in the central part: of the Aizu Basin after the deposition of Todera formation of early-middle Pleistcene inn age. On the other. hand, the middle course of the Agano River has cut down about. 100 140 meters during the same period, forming eight levels of fluvial terraces on the slopes of the river valley. The difference of these process is considered to be caused by crustal movements in both regions which have been divided by the active lineation in the western margin of the Aizu Basin.