Studies on the Geomorphological Features of the Fluvial Plains in Japan Focusing the Distribution, Geomorphological Land Classification and Its Application

Geographical Review of Japan Vol. 61(Ser. B), No. 1, 35-49,1988

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. Fuji is Fan+Natural levee (with Back-swamp)+Delta located at the crossing point of the two troughs. (Fig. 2). Because of the crustal movement mentioned above, the mountains of the central area are the highest in Japan. The landforms of the fluvial or coastal plains in Japan has been significantly influenced by geomorphological background.

III. Basic Form of the Fluvial Plain in Japan

The standard of the formation process of the fluvial plain is estimated as follows; firstly, the natural levee which consists of sand and gravels formed along the river course; secondly, new channels are formed partly because of the changes of discharge, partly because of the changes of the volume of sand and gravels transported by the rivers. And the new natural levees are formed along the new river courses. These natural levees consists of sand and gravels in the upper part of the plains and sand and silt or clay in the lower parts of the plains. The coefficient of adhesion of sand and gravel is less than that of sand, silt or clay. Therefore, the natural levees of the upper parts of the plain are very easily destroyed by the streams. Many streams will be formed in the Figure 2 Basic form of an alluvial plain upper parts of the plains and the shapes of the . l: gravel 2: sand 3: silt and clay 4: direc streams show braided courses. Lastly, the areas tion od flood water of sand and gravel i.e. in the upper parts of the plains became alluvial fans (OYA,1973). For example, in the Nobi Plain, which is located From the lower periphery of the fans to the in the lower reaches of the Kiso River, in the cen lower courses, the natural levees consists of sand. tral part of Japan, we see the large fan whose apex The coefficient of adhesion of sand is bigger than is Inuyama City, the diameter of which is 12km Geomorphological Features of the Fluvial Plains 39 and area is 100km2. And we see three or four tane depressions and gorges with knick points distinct natural levees from the fan to Nagoya (OYA,1977). City and Tsushima City. The line which connects Table 1 shows the geomorphological elements in Nagoya City with Tsushima City to south identi the "On" group, and Table 2 for "Od" group. fies the delta (Fig. 3). The features in each group are summerized as follows: "On" group: Nearly all plains are constructed by the fan, natural levee and back-marsh and the delta. The fan is generally big. The formation process of the fluvial plains of "On" group has already been discussed in chapter III. "Od" group: The fan is not found; if it is , it is small. The plain is mostly characterized by natu ral levee and back marsh and delta; some plains however have few natural levee and back marsh and delta. We now proceed to discuss the cause of the for mation of the features of "Od" group.

1. Whether or not the intermontane depres sions exist in the upper reaches deter mined the variation

(1-1) Mogami River (Fig. 4)

Figure 3 Geomorphologicalland classification map of the Nobi Plain (OYA,1956). 1. Mountain 6. Delta 2. Terrace 7. Reclaimed Land 3. Fan 8. Dry River Bed 4. Natural Levee 9. Tidal Limit 5. Back-Swamp 10. Area Inundated

IV. Regional Variation in the Combination of the Geomorphological Elements of the Fluvial Plain

The landform of the plain is influenced by the sand and gravels which has been transported from upper reaches and change of base level. Utilizing the geomorphological land classifica tion maps of plains in Japan, we have classified the fluvial plains into two groups: 1) "On" group which has no intermontane depressions in the upper reaches in the orogenic belt, and 2) "Od" group characterized by its intermon Figure 4 River system of the Mogami River Basin. 40 M. OYA et al.

Table 1. Combination of the geomorphological elements in the fluvial plain which has no intermontane depressions in the upper reaches in Japan "On" group (OYA, 1977).

F: Fan, VP: Valley Plain, N.L(B): Natural Levee and Back Marsh, D: Delta, S: Sand Dune, S. S: Sand Spits. Capital letter shows main strcam, Small letter shows tributary. The foot note is common to Table 2.

To identify the features we studied the lithol 3) The river course in the Arato Gorge and ogy, diameter and roundness of the particles of Yamagata Intermontane Depression; gravel (those of 64mm in diameter and larger) in 4) The river course in the Oyodo Gorge and the river bed of the Mogami River; we then singled Obanazawa Intermontane Depression; out those with the largest diameters and mea 5) The river course in the Shinjo Intermontane sured their roundness. Depression and The lithology and grain size frequency of grav 6) The Mogami Gorge and the Shonai Plain. els are shown on Figs. 5 and 6. These figures From the viewpoint of river bed sedimentation, show that sand and gravels are furnished in the the mainstream of the Mogami River consists of gorges and deposited in the intermontane depres the six different reaches where erosion and depo sions downstream. sition alternate. Based on the lithology of gravels, OYA (1974) The clear differences in the lithology and divided the Mogami River into six parts: diameter of the gravel of the six reaches show that 1) The Matsukawa River in the Yonezawa the depositional function of the intermontane Intermontane Depression; depression is very important. Due to the deposi 2) The Shirakawa River and the Main Mogami tion in the Yamagata, Obanazawa and Shinjo in the Nagai Intermontane Depression; intermontane depressions, no gravel of more than Geomorphological Features of the Fluvial Plains 41

Table 2. Combination of the geomorphological elements in the fluvial plain which has intermontane depressions and gorges with knick points in Japan "Od" group (OYA, 1977).

64mm in diameter flows down from the upper small fan was formed. reaches to the Shonai Plain via the Yamagata, Based on the above mentioned findings of the Obanazawa and Shinjo Intermontane Depres research of the river bed sediments in the Mogami sions. Gravels are only provided in the Mogami River and changes of the diameter of the largest Gorge. Because the absolute volume of the gravels gravels and its roundness in the Chikugo River which flow down to the Shonai plain is small, a (OYA, 1957) and Ishikari River (OYA,1958), we see 42 M. OYA et al.

montane depression as has been mentioned above. But regional differences exist in the landforms of gorges and intermontane depressions. Further more, the function is influenced by the gradient of the longitudinal profile.

(1-2) Shinano River The Shinano River in the central part of Japan is the longest in Japan. There are the Saku, Ueda, Nagano, Iiyama and Tokamachi Intermontane Depressions along the main stream of the river, and the Matsumoto Intermontane Depression along the Sai River, a tributary of the Shinano River (Fig. 7). The direction of the former group is SW-NE, i.e. crossing oblique with the island arc, and the latter group, SE-NW i.e. crossing in a right angle. We have investigated the river bed sediments and have found the phenomena des cribed below (NAKAYAMA,1975). There is no abrupt change of river bed sedi ments in the canyon between the Ueda and Nagano Intermontane Depressions, in the Tachigahana Canyon between the Nagano and Iiyama Intermontane Depressions and in the canyon between the Tokamachi Intermontane Depression and the Echigo Plain. The Akashina (between Matsumoto and Nagano Intermontane Depressions) and Kamisakai (between Iiyama and Tokamachi Intermontane Depressions) Canyons have long distances, respectively 60km and 40 km; on the other hand, the canyon between the Nagano and Iiyama Intermontane Depressions is short, only 10km. Furthermore, the width of the river in the former is about 100m, but the latter is 200m wide. Based on the above facts we conclude that even in the case of a river with intermontane depressions, if it is short and wide a considerable amount of sand and gravels will be allowed to flow down to the lower reaches without interruption in Figure 5 Species of the river bed sediments in the the intermontane depressions. Mogami River (OYA, 1974). 2. Relationships between the distribution of the intermontane depressions and that a considerable part of large gravels trans direction of island arc ported from the upper reaches is deposited in the intermontane depressions, while only sand and The features of the intermontane depressions, small-sized gravels are allowed to move to the gorges and longitudinal profile vary in North plain. Therefore, since the absolute volume of the eastern Japan, Central Japan and South-western gravels is small, a small fan was constructed. Japan. The function of the transportation of river bed In the case of N. E. Japan, the plains parallel the sediments is influenced by the gorge and inter island arc of Honshu. Therefore, the shape of the Geomorphological Features of the Fluvial Plains 43

Figure 6 Histogram of the river bed sediments in the Mogami River (OYA, 1974). plain is lean and tall and its area is wide. On the as compared to the two other rivers (Fig. 8). other hand, in the case of S.W. Japan, the plain is distributed separately and its area is small (2-1) Kitakami River because the river courses run in a right angular The fluvial plain along the Kitakami River is direction to the island arc. The longitudinal pro divided into two parts, Kitakami Intermontane file of the river is influenced by the distribution Basin and Sendai Plain, by the Kozenji Canyon. and area of the plains as mentioned above. Cen The longitudinal profiles of the Kitakami Basin is tral Japan and Shimanto and Hiji River in S. W. gentle around 1/1,000. The lower edge of the Japan as typical types. Please note that the gra basin, i.e. the area near directly upper part of the dient of the Kitakami River, running in parallel Kozenji Canyon, is made up of silt and clay (UHIDA, with the island arc, is the gentlest; the Shimanto 1985). River, running in a right angle with the island arc, The gradient of the Kitakami River in the is the steepest and the Shinano River is moderate North-Sendai is gentle, 0.1/1,000. The geomor 44 M. OYA et at.

ground movements caused by the push from the south. These tectonic ground movements have caused the distribution of the intermontane depressions and plains in Shikoku (TABUCHIand TADA,1965; ICHINOSE,1986). Fig. 9 shows the drainage system of the Shi manto and Hiji Rivers. Both the Hiji and Yoshino Rivers run between the two mountain ranges with a N-S direction, i.e. A-A', B-B'. The Yusuhara River run closely to the mountain range with a B-B' direction with incised meander (OTSUKA, 1927). The river flows down from north to south but does not enter the sea immediately, because of the existence of the mountain ranges with a direc tion with A-A'. In the case of the Hiji R., the Uwa Intermontane Depression is developed from north to the south.; the Nomura and Ozu Intermontane Depressions, WSW-ENE. In the case of the Shimanto River, the Hiromi and Nakamura Intermontane Depressions are developed from NW to SE. These depressions are developed in parallel with the river courses and in the depressed area. And the antecedent valleys in the Hiji and Shimanto River overlap the clossing point the main axes of the mountain Figure 7 River system of the Shinano River Basin. ranges. (The legend is the same as Fig. 4) As mentioned above, the topography of Shikoku is determined by the syncline and anticline axes. phological elements of the plain which consists of Due to the topography, the Shimanto River and are delta, natural levees, back-swamps and lakes Hiji River have many intermontane depressions such as Izunuma and Naganuma. and canyons repeatedly. This is one of the impor tant reasons why there is few plain in the outer (2-2) Rivers in Shikoku zone of S. W. Japan (NAKAYAMAand TOYosmMA, In Shikoku, there are topographic and geologic 1986). axes from west to east in parallel with the island 3. Classification of alluvial surface influ arc and the sub-axes are in a right angular rela enced by base level (sea level) tionship to the main axes, i.e. N-S. The main axes were formed by up-warping and down-warping As has been mentioned above, the influence of

Figure 8 Longitudinal profiles of rivers. Geomorphological Features of the Fluvial Plains 45

fication of the alluvial surfaces in the Tama River Plain. The Alluvial Surface I consists of fanic lowlands and flood plains characterized by the natural levees and back-swamps. There is no time lag between the two geomorphological units men tioned above because there is no knick point between the two geomorphological elements. UMITSU(1977) has studied the relationships between the alluvial surface and sea level changes as follows. The Alluvial Surface Ia and Ib were located at the lower reaches of the river during the period when the upper clay layer was deposited. There fore, we assume that the Alluvial Surface Ia and Ib were formed as continuous alluvial plain has been changed by eustatic movement. The Alluvial Surface II consists of deltaic or lagoonal lowlands, according to UMITSU(1977), the Alluvial Surdfce ha is a subaerial deposition, and IIb is a subaquatic deposition at the time cover the upper sand layer was formed. After this period, i.e. the formation of the upper most layer, the Alluvial Surface IIb became a subaerial deposi tion. This shows that the formation ages of the Alluvial Surface ha and IIb were different but the environment was the same. Based on the features of the alluvial layer and Figure 9 Basins and mountain ranges in south distribution of the alluvial surfaces, we feel that western Shikoku. the Alluvial Surface I was formed during the 1: mountain range 2: basin transgression in Holocene. And the Alluvial Sur face III was formed, correlated with the present the intermontane depressions and gorges is sea level and the Alluvial Surface IIIb was formed remarkable in the plains, especially fan and natu following the Meiji Restoration (1867) onward. ral levees. These intermontane depressions and On the formation age of the Alluvia] Surface II, gorges were formed by the crustal movements. UMITSU(1977) estimated that the upper sand layer But in the lower part of the plain, they were influ was deposited between the maximum transgres enced by the base level i.e. sea level. sion (6,000 years Bp) and 3,500 years BP. For example, TAKAGI(1975) researched the order of the alluvial surface in the plain of the Tama (3-2) Tsugaru Plain River Basin, Naka River Basin and Kuji River The Tsugaru Plain located in the N. E. Japan Basin in the Kanto Plain and UMITSU(1976) stu consists longely of alluvial fans in the south, a died the Tsugaru Plain in N.E. Japan. deltaic plain in the north and a natural levee zone in the middle. (3-1) Tama River Plain In the natural levees zone, the alluvial plains TAKAGI(1979) divided the Tama River plain into are divided into two levels which intersect each three groups: Alluvial Surfaces I, II and III based other; the upper alluvial plain dissected in its upon the knick points of the topography and the southern part and overlaid with deposits of the variation of the surface geology. new one in its northern part. Under the plain Fig. 10 shows the geomorphological land classi there are two terraces and fluvial valleys buried fication map taking into consideration the classi Holocene deposits. 46 M. OYA et al.

UMITSU(1976) describes the geomorphological history of the Tsugaru Plain in the Holocene period as follows; 1) In the early Holocene, former fluvial valleys were drowned by transgression and were filled with sediments. 2) with the progress of transgression, an embayment extended to the central part of the plain by 5,000 yBP. Drowned valleys had been entirely submerged and the expansion of sedimen tation area caused sediments to become fine. A delta composed of fine sediments was formed at the head of the embayment. 3) A slight regression caused dissection of the deltaic plain and new delta composed of rather coarse sediments was formed further downstream. 4) Around 2,500 yBP foreset flourished on the dissected deltaic plain, and peat accumulated on the new one. 5) After 2,000 yBP, the lower alluvial plain was formed burying forest and peat lands (Fig. 11). As has been mentioned above the lower part of the alluvial plain is influenced by the change of base level (sea water level).

The features of the geomorphological elements, the combination of the geomorphological elements and the order of the alluvial surface have close relationships with human activities i.e. land use, geomorphological hazards, etc. Lastly, we propose to discuss by utilizing the geomorphological land classification map, the prediction of the flooding and soil liquefaction caused by earth quakes.

Figure 10 Alluvial surfaces and micro-landform classification map in the lower Tama River alluvial lowland. Geomorphological Features of the Fluvial Plains 47

Typhoon. OYA(1956) completed a Geomorphologi cal Survey Map of the Kiso River Basin showing classification of f lood-striken Areas. Three years later since completing the above mentioned map, the area in this map was devastated by the high tide caused by the Ise-Bay Typhoon (Typhoon Vera) of 1959. And the result of the flood was just what this map anticipated (Fig. 3). Furthermore, we can use the map for identify ing the site of soil liquefactin caused by earth quake (OYAet al., 1982).

VI. Conclusion

As members of the Working Group on the Geo morphology of River and Coastal Plains in the Association of Japanese Geographers and lnterna tional Geographical Union, we have studied the geomorphology of the fluvial plain based on the landf orm order, especially the distribution of Figure 11 Holocene stratigraphy in the Tsugaru intermontane depressions and gorges in the upper Plain (UMITsu,1976). reaches, and geomorphological land classification and its application especially for geomorphological hazards. V. Application of the Geomorphological (1) The important factors contributing to the Land Classification to the Prediction formation of the plain are the volume and quality of the Flooding, etc. of the sand and gravels transported from the upper reaches. There are close relationships Each geomorphological elements in the plains between the volume and quality of sand and grav were formed by repetition of flooding. So the land els on one hand and the landf orm of the upper form of the plains shows the history of flooding. basins on the other. Then if we classify the topography of the plains as There are distinct regional differences in the geomorphological elements such as fan, natural landform of Japan. Based on the features of topo levee, delta etc,. We can foretell the type of flood graphy, we divided the Japanese Archipelago into ing in each geomorphologic element. For instance: four parts: S. W. Japan, Central Japan, N. E . Japan In the fan, the violent deposition, erosion and and Hokkaido. changes of the stream are often seen during flood In general terms, the mountain region is char time. In the natural levee, when this district is acterized by the upheaval, and the plains are submerged by flood, the water drains off speedy depressed. The plains have been formed by the and deposits sand. While in the back-swamp, the deposition of sand and gravels which were trans period of stagnation is longer and the water depos ported by the rivers from the mountains. its the silts. In the case of the following factors (2) Basically, the combination of the geomorpho the fan, the natural levee, the back-swamp and logical elements of fluvial plain is: the delta-have a decisive influence upon deciding Fan+Natural levee (back-marsh)+Delta. the type of flood. This shows that in the case of The geomorphological elements were formed by the alluvial plain, the flood type may be traced the repetition of flooding. according to the micro-topography of the plain. One of the typical types of the plains is the Nobi This relationships were proved by a geomorpho Plain in the Central part of Japan. logical survey map of the Nobi Plain showing the (3) Distinct regional differences can be identi classification of flood striken areas and the fied in forms of the combination of the geomorpho 48 M. OYA et al. logical elements. Zone of SW. Japan. NAKAYAMA,M. ed., On the Fea The river which has intermontane depressions tures of the Geomorphology of the Plain in the Outer and gorges with knickpoint in the upper reaches Zone of S. W. Japan Having a Lot of Rainfall. Report of the special fund of the Ministry of Education, 26-37. (J) significantly contributes to the regional differen NAKAYAMA,M. (1985): On the Relationships between the ces. A considerable part of the large size gravels Features of the River Bed Sediments and Longitudi transported from the upper reaches is deposited in nal Profile of the Shinano River. Miscellaneous Report the intermontane depressions, while only sand on the 50 years Anniversary from the foundation of the and small-sized gravels are allowed flow down Geographical Institute, Nihon University -Kanto Dis trict and its Vicinity-345-352. (J) ward to the plain. Therefore, when the absolute NAKAYAMAM. and TOYOSHIMA,Y. (1986): On the Fea volume of gravels was small, a small fan was con tures of the Landform Order in the Western Part of structed. This phenomenon has been clarified by Shikoku and Southern Part of Kyushu. NAKAYAMA,M. several studies of river bed sediments in the ed., Features of the Alluvial Plain in the Outer Zone of Mogami River and other rivers. S. W. Japan Having a Lot of Rainfall. Report of the special fund of the Ministry of Education. 2-14. (J) Regional differences are also noticeable in the OKAYAMA,T. (1974): Topography of the Mountains in functions of gorges and intermontane depressions Japan. Kokonshoin. 43-80. (J) in the upper reaches. The differences were OTSUKA,Y. (1927): A Study on the Meander along the formed by the distribution, landform of gorges and Shimanto River. Geographical Review of Japan, 3, intermontane depressions and their gradients. 396-419. (J) OYA,M. (1956): Reconnaissance Topographical Survey on Whether or not the river flows in parallel with the Areas Subject to Flood: A Topographical Survey Map of island arc has a decided influence on these the Kiso River Basin Showing Classification of Flood features. Striken Areas. (J) Furthermore, the landform of the plain in the OYA,M. (1957): On the Velocity of the River-flow in the lower reaches was influenced by the fluctuation of Mountains, Basins and Canyons in the case of the River Chikugo, Kyushu. Geographical Review of Japan, base level (sea water level). 30, 21-34. (J-E) (4) The geomorphological elements such as fan, OYA, M. (1958): Fluvial Function of Kamikawa Inter natural levee, back-marsh and delta, and their montane Depressions and Kamuikotan Gorges in the combinations show the history of flooding of the Ishikari River. Resources Bureau, Science and Tech nics Agency. Relationships between the topography of plain. Therefore, by making a geomorphological Peat Land and Flooding. (J-E) land classification map of the plain, one can pre OYA, M. (1973): Basic-form of the Combination of the dict the feature of flooding in the future. The Geomorphologic Elements in the Alluvial Plain. The accuracy of the geomorphological land classifica Scientific Researches, Scool of Education, Waseda Uni tion map was proved by the Typhoon of Vera (so versity, 22. 23-43. (J-E) called Ise Bay Typhoon) of 1959 at the Nobi plain. OYA,M. (1974): Fluvial Function of Intermontane Basins and Gorges in the Mogami River, Northeastern Japan. Furthermore, we can use the map for identify Annals. of Tohoku Geographical Association, 26, ing the site of soil liquefaction caused by 123-129. (J-E) earthquakes. OYA, M. (1977): Comparative Study of the Fluvial Plain Based on the Geomorphological Land Classification. Acknowledgement Geographical Review of Japan, 50, 1-31. (E) OYA, M., KOTODA, K., WAKAMATSU,K, and KUBO, S. (1982): Geomorphologic Land Classification Map of the We thank Dr. Yung PARK,Professor of California Shonai Plain Illustrating Features of Floodingand Soil State University and Visiting Professor of Waseda Uni Liquefaction. Tohoku Regional Construction Bureau, versity, who so kindly looked over our papers. We also Ministry of Construction. (J-E) express their profound thanks to Miss S. KUBO,assist TABUCHI,H. and TADA,F. (1965): Geographical, geologi ant of Waseda University, who cartographed and typed cal research. Report on the Countermeasure of the our paper. Prevention Disaster. Taisho Forestry work Office. (ReceivedDecember 8, 1987) Kochi Regional Forestry Bureau. (J) (AcceptedMarch 5, 1988) TAKAGI, I. (1975): Relationships between the Alluvial Surface and Joni Grid Pattern System of the Lower References Reaches of Rivers in the Kanto Plain. Miscellaneous Report on the 50 years Anniversary from the foundation ICHINOSE,Y. (1986): The Relationships between the of the Geographical Institute, Nihon University Flooding and Formation of the Plain in the Outer Kanto District and its Vicinity 91-101. (J) Geomorphological Features of the Fluvial Plains 49

TAKAGI,I. (1979): The Regional Distribution of Alluvial Tsugaru Plain in the Holocene Period. Geographical Surfaces in Japan. Proc. of the Institute of Natural Review of Japan, 49, 714-735. (J-E) Sciences (Geography), Nihon Univ., 14, 21-30. (J-E) UMITSU, M. (1977): Paleogeographic Reconstruction of UCHIDA,K. (1985): Retarding Reservoir and Flood Control the Tama River Plain, by Means of Mesh-map Study on the Applied Geomorphological View Point. Method. Geographical Review of Japan, 50, 596-606. Kokonshoin.126-163. (J) (J-E) UMITSU, M. (1976): Geomorphic Development of the

地形配列,地形分類を基礎とした日本の河成平野地形とその応用に関する研究

大矢雅彦*・中山正民**・高木勇夫***

(1)平野地形をつくるもっとも重要な要因は上流かかなりの部分が盆地で堆積してしまい,小粒径のものだら運搬されてくる砂礫の質,量である.これは流域の地けが峡谷を経て流下する.このため扇状地はないかあっ形,地質と密接な関係をもっている.この地形,地質にても小規模となる.これらの事を最上川など数河川の河は著しい地域差がある. 床砂礫の粒度分析でたしかめた.盆地・峡谷の機能はそ一般的に日本は山地地域は地盤の隆起を ,平野は沈降の形態,規模,縦断勾配などによって異なる.これらのをくりかえしてきた.そして平野地形は河川が山地より相違は河川の流向が島孤に対して平行であるか,交叉す運搬してきた砂礫によって形成されてきた. るかによって影響を受けている.また,海岸部では沖積 (2)平野における基本的な地形要素の組合せは次の面は基準面(海面)変動の影響をうけている. ようになる. (4)沖積扇状地,自然堤防,後背湿地,デルタなど扇状地+自然堤防(後背湿地)+デルタの地形要素とその組合せの特色は洪水の歴史を示すものこれらの地形要素とその組合せは洪水の繰返しによっである.それ故平野の地形分類を行えば将来万一洪水氾て形成され,その代表例は中部日本の濃尾平野である. 濫があった場合の洪水の状態の予測が可能のはずであ (3)しかし,実際には沖積平野の地形要素の組合せる.水害地形分類図の価値は濃尾平野をおそったべラ(伊には著しい地域差がある.この地域差をもたらした重要勢湾)台風によって立証された.この他地形分類図は地な原因は上流における盆地及び遷急点をもった峡谷の有震の際の液状化地点の予測にも利用できる. 無である.盆地があると上流から供給された砂礫のうち,

* 早稲田大学教育学部東京都新宿区西早稲田1-6-1 ** 埼玉大学教育学部埼玉県浦和市下大久保255 *** 慶応義塾大学経済学部東京都港区三田2-15-45