The working group on “Geomorphology of River and Coastal Plains” of the Association ofJapanese Geographers was founded in 1979. Its members have presented 85 studies during the last 10 years. The main features of these studies can be summarized as follows: 1) Almost all of the studies have been based on geomorphological land classification. 2) We have paid attention not only to the geomorphology of alluvial plains but also to the overallcondition of drainage basins. In particular we have discussed the influence of catchmentcharacteristics on the topography of alluvial plains. Most notable is the major influence of anintermontane depression in the upper reaches on an alluvial plain in the lower reaches. 3) We have paid attention to the relationships between micro-topography of the plains andarchaeological sites such as shell mounds, etc. 4) The coasts of Japan have been divided into several groups on the basis of environmentalcharacteristics such as landforms, vegetation, hydraulic conditions, and climate. 5) Applied geographical studies have been carried out using geomorphological land classificationmaps. These maps have been used in the identification of areas subject to inundation and soilliquefaction due to earthquakes. 6) Integrated regional studies have been carried out in contrasting climatic zones such as theGkhotsk sea coastal plain, in the subpolar zone and the outer zone of S. W. Japan in the humidtemperate zone. 7) We have compared the research on alluvial plains carried out in the stable continental zones ofother cuntries - for example, the Korean Peninsula and the Central Plain of Thailand - withthat on Japanese plains. We have planned this special publication on “The Geomorphology of River and Coastal Plains inJapan” based on the evidence collected in these studies.
We have conducted a comparative research on the fluvial plain between Japan and South Koreabased on geomorphological land classification. There are distinct regional differences in thegeomorphology of Japan and South Korea even though they are adjacent to each other. The fluvialplains in Japan are depositional plains, but in South Korea they are basically erosional plains, eventhough they are thinly covered with sand and gravels. Based on the combination of geomophological elements, the plains of Japan and South Korea areclassified as follows: <Japan> (“On” group =basic form) Fan+Natural Levee (Back Marsh)+Delta. (“Od” group) small fan+small natural levee+Delta no fan+no natural levee+Delta. <Korea> (basic from) Pediment+Pen-pediment+Natural Levee (Back Marsh)+Tide land. (R. Naktong) Pediment+Peri-pediment+Natural Levee (Back Marsh)+Delta+Tide land. The above outlined differnces were formed by the following factors: 1) In Japan, erosion in the mountainious region is pronounced due to the steep topography andbecause of the torrential rainfall. Sand and gravels were supplied to the valley bottom by land collapses and transported to the plainby rivers during the flood season. They were deposited at the boundary between the mountainousregion and plains, and formed fans. In South Korea, a great deal of fine-grained material is produced because of the wide distributionof gneisses and granitic rocks which are easily weathered and by mechanical weathering in themountainous region due to the low temperatures during the winters. The debris which was formed onthe surface of the mountanious regions was washed away by torrential rainfall during the summerseason. The gravels were deposited on the gentle piedmont slopes, especially the lower parts ofpediments which were thinly covered with sand and gravels were formed as peri-pediments. 2) The coefficients of river regimes are large in the rivers of both countries. The ratio in Korea islarger than that of Japan. The longitudinal profile of rivers in Japan is steeper than that of Korea. Due to the above mentioned features of the rivers, a great deal of sand and gravel was transportedin Japan and sand were transported in South Korea, so that natural levees are developed better in South Korea than in Japan. 3) In Japan, the deposition of sand and gravels a t the lowest reaches is remarkable. Because manyrivers pour into inland bays, many big deltas are formed in the lowest reaches. In South Korea, the deposition of sand and gravels in the lowest reaches is difficult partly becausethe rivers pour into the open sea directly, and partly because of the very wide tidal range, for example 8.1m at the lowest reach of the Han River. 4) The differences between the two groups based on the combination of geomorphologicalelements in Japan are related to whether a river has intermontane depressions and gorges in the upperreaches or not: the “On” group has no intermontane depressions or gorges in the upper reaches, whilethe “Od” group has. The differences in the combination of geomorphological elements in Korea are based on naturalconditions at the river mouth.
The Outer Zone of S. W. Japan consists of Kui Peninsula, Southern Shikoku and Southern Kyushu.The purpose of this paper is to describe the distribution of intermontane basins and coastal plains inrelation to the up and down warping and volcanic activities during the Quaternary. The distribution of intermontane basins and coastal plains, with its marked regional and localvariation, mainly shows the different varieties of upheaval movements. There are few intermontanebasins in Kui Peninsula, the east and the north of the mountains of middle Shikoku, and in the KyushuMountains, where upheaval movements are rather small. There are topographic and geologic axes extending from west to east, in parallel with the MedianTectonic Line and the island are in Outer Zone. Sub-topographic axes are at right angles to the mainaxes; i.e. N-S. The main axes were fromed by gentle up and down warping by a push from the south. Thedistribution of intermontane basins and coastal plains in western Shikoku and eastern Kyushu havebeen controlled by the slow up and down warping. When a river has intermontane basins, a considerable part of the large, heavier gravel which istransported from the upper reaches is deposited in the basins while only sand, silt, clay and small-sizegravel is allowed to continue on its way. This is one of the important reasons that there are few wideplains in the Outer Zone. The eruptions of Quaternary volcanoes have influenced the basin floor and coastal Landformdevelopment. We have found volcanic ash formation (Akahoya ash which dates back to 6, 300 y. B. P.) in the boring cores of the coastal bay head alluvial plains in the Outer Zone. Ash layers overlaymarine silt and clay formation with a thickness of 1 to 9m.
最終氷期以降に形成された日本列島の海岸砂丘の地形,形成期および古土壌の生成環境・母材の起源について考察した. 日本列島は,太平洋岸の新・旧砂丘分布域と,日本海および東シナ海沿岸の新・旧・古砂丘分布域に二分され,砂丘砂は非石灰質砂,石灰質砂,火山灰質砂などからなる.完新世の砂丘は,約6,000y. B. P.の海面高頂期に続く小海退初期に形成された後,海水準微変動に伴って並列型,複合型,累積型の砂丘列が形成された. 2,000y. B. P.以降になると地形改変の進行につれて流出土砂が増大し,それまでの砂丘形成の図式が変化し,とくに藩政時代から砂丘の大型化が顕著になった.この時期に形成された二次砂丘と風食窪の長軸方向は,現在の卓越風向 (ベクトル)とほぼ一致する. 最終氷期の古砂丘は, 7万, 5万, 3万, 1.8万y. B. P.に形成され,このうち7万y. B. P.と5万y. B. P.の古砂丘は広域に分布する.南西諸島のこう (膠)結砂丘も同じく氷期に形成されている.現在のクロスナの生成は,後浜からの飛砂をくい止める一次砂丘背後の植生で被われた砂丘上でみられる.クロスナや古土壌中の細粒物質は,中国大陸や陸化した大陸棚,砂浜起源の風成塵が主なものである.