Fluvial Landforms in River Plains in the southeastern Part of Aomori Prefecture

Abstract

The river plains in this area are divided into the Holocene fluvial terraces and the flood plains. In the northern part of this area, the river plains of the mid- and down-stream of the Aisaka and the Gonohe Rivers are among the uplands, and the width of the river plains deviates a little except at the river mouths. Near the river mouths, there are old meander channels, but only a small number of natural levees are distributed. The natural levees are generally so low and flat as ones near the mouth of the Mabechi River to the south. In the reach about 3 to 5 km from the river mouth, the flood plain is smaller than ones in the upstream and at the river mouth. The width varies a little, and the surface of the terrace is flat, composed of peaty clay and silt. Further, in the reach about 5 to 15 km from the river mouth, the natural levees composed of silt and sand continue on the fluvial terrace, higher than those of the downstream. Larger ones are about 2 m in relative hight above the fluvial terrace. The terra e surface behind the natural levees is more undulated, composed of peaty silt and clay. The flood plain composed of sand and gravel below this fluvial terrace has wider meander belt, and many well-developed old channels.From the above geomorphological evidences, the processes of the rivers may be concluded as follows.Just after the completion of the Holocene fluvial terrace surface, the rivers were gentle, and well meandered, and formed continuous higher naturl levees. In time, the rivers deepened, and formed the valleys under the present flood plains according to the lowering of the base level. Burying the valleys with fluvial sediments, the rivers on the flood plains gradually became gentle, developed meander channels again, and formed lower natural levees. At present, the rivers are deepening a little.