抄録
The formation of floodplain can be broadly grouped under two obvious stream environments : channel deposition and overbank deposition. Point bar and channel floor etc. are the landform as the result of the channel deposition. Natural levee and back marsh etc. are as the result of the overbank deposition. Many foreign scholars emphasize that the floodplain mainly consists of the channel deposits and the proportion of overbank deposits is generally very small. There are few studies of this kind in Japan. Then the author selected the lower Kinu River Floodplain as the study area where is situated in the central part of Kanto Plain. He has examined the relationship between the landforms and those deposits in order to discuss the formation of floodplain in Japan. 1. There are natural levees back marshes clevasses, point bars, former river courses and river sand dunes etc. in the floodplain. Particularly the natural levees and the back marshes are developed widely. The point bars are seen much confined along the Kokai River. 2. The author digged the 4m holes with the hand auger at many places in the floodplain, and made the grain size analysis of the sediments. The natural levees are composed of the sand, clayey sand and loam within the depth of I m, and of the clayey silt and clay below that depth. The back marshes are made up of silty clay and clay. The point bars are formed of sand. The crevasses have a depth of I m and composed of sand. In short, in this floodplain the present landforms correspond directly to the deposits within the depth of I m. 3. According to the observation of the deposit faces, the author has found following facts: a. The peat layers are broadly distributed and deposit thickly. b. The peat layers or the back marsh deposits are often covered with the sandy deposits in the natural levees. c. The point bar deposits are confined along the meander sections by the Kokai River. Judging from the facts mentioned above, it is said that the overbank deposition prevails in this floodplain and the overbank deposits occupy the most part of the surface geology. 4. Based on the radiocarbon date of the peat and the age of the unearthed potteries, it is thought that there was a good development of the natural levees around thirteen century in this floodplain.
