Geographical Review of Japan Vol. 61(Ser. B), No. 1, 35-49,1988 Studies on the Geomorphological Features of the Fluvial Plains in Japan Focusing the Distribution, Geomorphological Land Classification and Its Application Masahiko OYA*, Masatami NAKAYAMA** and Isao TAKAGI*** (1) The geomorphological features of the fluvial plain are strongly influenced by the volume and quality of sediments transported from the upper reaches of the river. There are close relationships between the sand and gravels on one hand, and the landform of the upper basin on the other. Generally speaking, the mountain region is characterized by its upheaval and the plains are depressed. The plains were formed by the deposition of sand and gravels which were transported by rivers from the mountains. (2) Basically, the combination of the geomorphological elements of fluvial plain is: Fan+Natural levee (back-marsh)+Delta. The geomorphological elements were formed by the repetition of flooding. One of the typical types of the plains is the Nobi Plain in the Central part of Japan. (3) Distinct regional differences can be identified in forms of the combination of the geomorphologi cal elements. The river which has intermontane depressions and gorges with knick point in the upper reaches significantly contributes to the regional differences. A considerable part of the large size gravels transported from the upper reaches is deposited in the intermontane depressions, while only sand and small-sized gravels are allowed flow downward to the plain. Therefore, when the absolute volume of gravels was small, a small fan was constructed. This phenomenon has been clarified by several studies of river bed sediments in the Mogami River and other rivers. Whether or not the river flows in parallel with the island arc has a decided influence on these features. The plains along the sea coast was influenced by the fluctuation of sea water level. (4) The geomorphological elements such as fan, natural levee, back-marsh and delta, and their combinations show the history of flooding of the plain. Therefore, by making a geomorphological land classification map of the plain, one can predict the feature of flooding in the future. The accuracy of the geomorphological land classification map was proved by the Typhoon of Vera (so-called Ise Bay Typhoon) of 1959 at the Nobi Plain. association with the Working Group on the Geo I. Activities of the Working Group morphology of River and Coastal Plains, IGU on the Geomorphology of River which is also chaired by M. OYA,vice chairman is J. and Coastal Plains in Japan SZUPRYCZNSKI(Poland), and the number of corres ponding members is about 150. The Working Group on the Geomorphology of The purpose of the Japanese Working Group is River and Coastal Plains was founded in 1979 to undertake studies of the geographical and within the Association of Japanese Geographers. applied geomorphology of the rivers and coastal It is chaired by M. OYAwith M. NAKAYAMAand I. plains in Japan. TAKAGIserving as advisory members. It has more The plains in Japan are small in size and occupy than 60 corresponding members. only 15% of the whole country, but they are very The working group carried on its activities in important because more than 80% of the Japanese * Faculty of Education , Waseda University, Shinjuku-ku, Tokyo 160 Japan. ** Faculty of Education , Saitama University, Urawa-shi, Saitama Pre. 338 Japan. *** Faculty of Economy , Keio Gijuku University, Minato-ku, Tokyo 108 Japan. 36 M. OYA et al. people live in the plains and all cities which have nary Period. The thickness of the sand and gravel population of more than 500,000 each are located layer of the Holocene, Pleistocne and Tertiary is in the plains. about 2,600m in the eastern part of Tokyo. The working group has examined the plains not Both the fluvial and coastal plains in Japan are only from general physical geographic but also distinct geomorphological differences. These regional and applied geographic viewpoints. regional differences were caused by natural fac Many studies are available the landform of Japa tors especially geology and landforms. nese river and coastal plains. The investigated From a geologic view point, the Japanese aspects include the structure and agents of the Archipelago can be divided into six parts: S. W. plains and relations between topography and eus Japan and N. E. Japan divided by the Fossa Magna; tatic movements. In this paper we wish to pay S. W. Japan divided into the Inner Zone (Japan Sea special attention to the studies of the plains based side) and Outer Zone (Pacific Ocean side) by the on the geomorphological land classification and its Median Tectonic Line; N. E. Japan is divided into application. the Inner Zone (Japan Sea side) and the Outer At the outset, we propose to discuss: Zone (Pacific Ocean side), and the Mainland of 1) the river system and landform of the basin in Hokkaido by the Morioka-Shirakawa and Sapporo the upper reaches which have close relation Tomakomai Lines (Fig. 1). ships with the alluvial plain; The Fossa Magna Region is a depressed area. 2) the basic form of the fluvial plain; But the area is covered by volcanoes at present. 3) regional variation in the combination of the Mt. Fuji is located in the region. The crustal geomorphological elements of fluvial plain; movement of the Outer Zone of S. W. Japan shows whether or not the intermontane depressions warping. There are few plains in this region. The exist in the upper reaches determing these crustal movement of the Inner Zone of S. W. Japan variation; shows a block movement. Each block is limited by 4) the order of the alluvial surface; fault lines. There are many plains and intermon 5) application of geomorphological land classifi tane depressions among the horst mountains. Also cation map to study of geomorphological there are few plains in the Outer Zone of N. E. hazards. Japan. But there are many plains and intermon tane depressions on the Inner Zone of N. E. Japan. II. The Brief Description of the Framework The height of mountains in Hokkaido are gener of Landform of Japan ally low except Hidaka Mts, and their slope is gen tle and the plains are wide because of the moder The Japanese Archipelago is situated in the Cir ate crustal movement and severe weathering in cum Pacific Orogenic Belt. Thus the most strik the glacial age. ing features of Japanese landforms are mountain From a geomorphological view point the ous regions which make up her land. Relatively Japanese Archipelago can be divided into four large plains include the Kanto Plain, the Nobi parts: S. W. Japan, Central Japan, N. E. Japan and Plain and the Echigo Plain in Honshu, the Ishikari Hokkaido (OKAYAMA,1974). Plain in Hokkaido, and the Tsukushi Plain in The Japanese Archipelago consists of several Kyushu. And many small plains and intermon island arcs. The coast of S. W. Japan on the Japan tane depressions are distributed along the river Sea side is of a convex type; on the Pacific side is courses (Fig. 1). concave. And the major mountain ranges, Chu The crustal movement of the Japanese Archipe goku Mts. and Shikoku Mts., are situated in paral lago is remarkable. Generally speaking, the lel with the coast line. They are not high, usually mountainous region is characterized by its uphea less than 2,000m. val, and the plains region is depressed. For exam On the other hand, in the case of N. E. Japan the pie, the mountains in the central part of Honshu Pacific coastal lines are convex but concave on the has been uplifted remarkably more than 1,500m, Japan Sea side. The main mountain range Kita on the other hand, the area of Kanto Plain was kami, Abukuma, Ou and Dewa Mts. run in paral depressed more than 1,000m during the Quater lel with these coastal lines. The height of the Geomorphological Features of the Fluvial Plains 37 Figure 1 Topographic map of Japan. 38 M. OYA et al. mountains tends to be less than 2,500m but there that of the sand and gravel layers so the bank are many volcanoes in the Ou and Dewa Mts. erosion is smaller in the lower parts than that of In the case of the central part of Japan, the the upper parts of the plains. The stream course mountain ranges do not parallel the coast lines. is stable showing the meander. Natural levees are For example, the Hida (Northern Japan Alps) and developed well along the river courses. Back Kiso (Central Japan Alps) and Akaishi (Southern swamps occupy the spaces between natural Japan Alps) run N-S or NNE-SSW. And the levees. The back-swamps consists of silt or clay . Kanto Mts. runs NW-SE. The height of the Along the sea-coast, natural levees become smaller mountains is very high, some as high as 3,000m. because the area is influenced by marine action And there are volcanoes also. i.e., ebb and flow and high tide caused by low According to a study by OKAYAMA(1974), the atmospheric pressures-Typhoons. We call these directions of the high mountain ranges were areas deltas. In this region, the slope of the formed by the push from the south and changed streams is straight and they make deep trenches from W-E to SW-NE and N-S to SE-NW. Two in the plains. The deltas consists of silt and clay. marine troughs, Sagami Trough from SSE to Namely, the standard form of the fluvial plains is NNW and Suruga Trough from SSW to NNE, as follows: shows the push from the south, the Mt.