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.

View full abstract

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.

View full abstract

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.

View full abstract

Japan is one of the few countries in the world affected by a comparativly wide variety of coastalenvironments.
There are regional differences in the distributions of coast-depositional topographies and beachsediments. These reflect the facts (1) that the Japanese Islands cover the “three major shore processzones” (Davies, 1972, 1980), and (2) that they are situated in a “tectonically active and climatically humid zone”.
The environments affecting coast-depositional topographies are classified on the basis of followingthree scales;
The first-scale environments are global and show peculiar regional characteristics of the Japanesecoast from a global point of view. They include (1) the “three major shore process zones” and cold, mild and subtropical climates. (2) the tectonic zone, and (3) a humid climate in the east EurasianContinent.
The second-scale environments show distinctive characteristics in each coastal region. Thesecond-scale environments include (a) winter pack ice and a cold climate in the high latitide coastzone, (b) a temperate climate and storm waves caused by the passage of typhoons and lowatmospheric pressures in the mid latitude coast zone, (c) a subtropical climate, storm waves caused bythe passage of typhoons, the Kuroshio Current, and the development of coral reefs in a low latitudecoast zone, (d) intense rainfalls, (e) volcanic activities, uplift movements, and subsidence, and (f) thenorthwesterly winter monsoon.
The third-scale environments include storm waves, prevailing winds, kinds of vegetation, rate ofdenudation in mountainous areas, and soon, for each coast.
These scales correspond to “global”, “national” and “regional” scales of Nakayama (1982).
Some of these phenomena, however, inreract with other phenomena. Larger environments affect those of the next scale down.
The author divided the coast of Japan based on the following orders: The first order division ismainly based on the “three major shore process zones” by Davies (1972, 1980). The second-orderdivision is based on the differences in the sea conditions. These two divisions produce Japan'sregional varieties of the coast-depositional topographies. The third-order division is based on thedevelopment of long beaches more than 20km. The fourth-order division is based on the combinationof beach materials and the topography behind the beaches. The additional division shows whetherdunes are stable at present or not.
In conclusion, based on the above mentioned division, the Japanese coast is divided into 14 different coastal areas (fig. 6).
There is a systematic relationship between an environment and a coast-depositional topography.Together, they will be called the “topography-environment system”. This system defines not only theregional and seasonal characteristics of shore processes, but also the regional characteristics ofcoast-depositional topographies in individual regions and in Japan as a whole (Fukumoto, 1987b).
From a global point of view, the environments which affect the formation process of thecoast-depositional topography in Japan is peculiar in the world.

View full abstract

The sandy coast of Japan is characterized by large-scale dunes which were formed during the Holocene and the late Pleistocene periods. Coastal dunes are composed of eolianite of aqueous origin, noncalcareous sand, and volcanic sand. Eohanite is widely distributed in the southern part of Japan, while non-calcareous sand is more common in coastal areas. Dunes consisting of volcanic sand are developed along the coasts near volcanos (Fig. 1).
The results of the present research may be summarized as follows:
1) Data presented in a study of Holocene dunes by Endo (1969) indicate that older dunes (Do) were formed during 6, 000-4, 000 y. B. P., while younger dunes (Dy) were formed during 1, 800-500 y. B. P. In some coasts, dunes were formed during 3, 500-3, 000 y. B. P.
Holocene dunes developed mainly as a result of Holocene sea level oscillations. As eolian sand blew up from expanded sandy beaches in the initial stage of regression following transgression, coastal dunes were formed on the beach ridges. The sequence of events that led to the formation of dunes was repeated at least four times during the Holocene age. As a consequence, three types of dune lines, as shown in Fig. 5, have been established in conformity with the shape and gradient of the coastal plain.
Since 2, 000 y. B. P., however, the increase of sand supply on the coast has resulted in changes in the dune formation mechanism, depending on land transformation in the inland area (Fig. 6). In particular, extensive land transformation since 1, 300 y. B. P. accelerated sand supply on the coast. Moreover, tree-cutting for salt-making and forest fires on the dunes have resulted in large-scale formation of secondary dunes. Consequently, the orientation of the Holocene secondary dunes coincides with the direction of prevailing winds stronger than 4. 5m/sec (Fig. 3). Landesburg's method can be adopted satisfactorily to show the relation between prevailing winds and dune orientation in Japan.
2) During the last Glacialage, paleodunes developed extensively along the coasts of the Japan Sea and the East China Sea. These paleodunes contain several datable paleosols which divide individual dunesand members into four parts. The four major periods of dune formation are as follows: 18, 000 y. B. P., 30, 000 y. B. P., 50, 000y. B. P., and 70, 000 y. B. P. The latter two periods, which saw dune formation over a wide area, are associated with the early stage of regression during the last Glacialage. The distribution of dunes of 18, 000 y. B. P. and 30, 000 y. B. P. is limited to a few coastal areas.As coastlines in these two periods were located more than 50km from the present coastline, sandcould not have been transported by prevailing winds to the position of the present coast line.
Lithified dunes with several sequences of eolianite and brown paleosol associations are found in the Ryukyu Islands. It is recognized that lithified dunes developed all over the world during the last Glacial age. Those of the Ryukyu Islands, were formed after 30, 000 y. B. P.
3) Kurosuna paleosols of the Holocene age were buried in the Holocene dunes. Thick modernhumus layers appear on stabilized dunes which are covered with shrubs at the back of foredunes along the coast of northern Hokkaido (Fig. 7). By analogy with the present condition of soil development in dunes in northern Hokkaido, kurosuna developed in stabilized inland dunes if the foredune could hold eolian sand from the sandy beaches.
4) Five paleosols of the last Glacial age are observed in the paleodunes. The clay assemblage of paleosols consists mainly of quartz, 14A minerals, and illite which originated in eolian dust from Continental China, dried sea floors, and sandy beaches. The oxygen isotope composition of fine quartz in the paleosols, as well as kurosuna, also reveals that fine quartz originated in eolian dust (Fig. 8).

View full abstract

In the alluvial lowlands along the seacoast in Japan, there are many maritime coastal lakes (Fig. 1). It is important that we have precise knowledge about these lakes in order to understand the development of the alluvial lowlands. The purpose of this paper is to clarify the geomorphological features of the maritime coastal lakes in Japan and to consider their geomorphological development.
The number of maritime coastal lakes in Japan is 31, a majority of the 51 lakes whose are as aregreater than 4km2. The significant feature of these lakes is that the maximum depth of each one is very small compared with its area (Fig. 2). Along the shoreline of a maritime coastal lake, we can generally find lacustrine lowlands, which develop at an altitude of 0-5m and at widths of 200-700m, and also a littoral shelf whose depth is only a few meters.
The lakeside lowlands are composed of three different parts: that is, lacustrine terrace I (at an altitude of 2-5m, width of 100-200m), lacustrine terraae II (at an altitude of 1-2m, width of 100-500m), and present sand beach or littoral marsh (at an altitude of 0-lm). Both the lacustrine terraces may have been formed at two different high lake water levels in the past, and have been connected with high sea levels in the.Holocene. It is assumed that lacustrine terrace I was formed in the period of the “Jomon transgression” (about 6, 000 y. B. P.), and lacustrine terrace II was formed in the period of the “Jomon re-transgression” (about 4, 000-3, 000 y. B. P.) or in the Heian era about 1, 000 years ago.
On the other hand, the littoral shelf can be divided into two other terraces, based on the differences in depth, sediment, and shape (Fig. 4 and Fig. 5). Littoral shelf I develops widely and continuously at a depth of 0.5-2.0m, width of 200-300m. It is certain that littoral shelf I is now under construction at the present lake water level. But littoral shelf II is supposed to have been formed as a topset surface of a river delta, a tidal delta, a sand bar, a sand spit, or a wave cut bench, at a low lake water level in the past. This low lake water level is assumed to be correlated with the low sea level whichappeared three times after the maximum transgression in the Holocene; that is, about 4, 500 y. B. P., 3, 000-2, 000 y. B, P., and in the 16-17th centuries. The present sand beach and littoral shelf I tend to develop around the widespread lacustrine terrace II, where lacustrine terrace I often remains conspicuous. Fhis fact suggests that the present landforms develop under the strong influence of past ones.
Finally, the author would like to discuss the issue from the applied geomorphological point of view. Recently practical use of the maritime coastal lakes has come to be emphasized. But many kinds of human activity-for example, construction of a littoral bank, a large-scale reclamation, or artificial control of the lake water level-have affected the natural ecosystem and traditional land utilization by an area's inhabitants. We should first pay attention to the geomorphological features of maritime coastal lakes and should estimate accurately the influence of human activities, in order to make the best use of such lakes.

View full abstract

Most coastal lowlands in Japan are characterized by the presence of barriers which are elongated sand or gravel ridge complexes usually parallel to the shore. The barriers are considered to have played an important role in the geomorphic evolution of the coastal lowlands during the Holocene.
Coastal barriers are classified into “primary” and “secondary” ones. A primary barrier was formed during marine transgression and migrated landward as the sea-level rose. A secondary barrier was formed seaward after the primary barrier had reached its most inland position. A secondary barrier usually develops as a series of beach ridges.
Geomorphic evolution of coastal lowlands has been deeply influenced by the Postglacial sea-level change. However, the geomorphic development of coastal lowlands with barriers is still not well understood. In particular, we have not done enough studies on the development of a bay or a drowned valley formed during the Postglacial transgression in the period of the first half of theHolocene.
The purpose of this paper is to clarify the geomorphic development of coastal lowlands with barriers in the Holocene by reconstructing the environmental changes of the bays or drowned valleys on the basis of analysis of the fossil foraminiferal assemblages in the bore hole cores. The paper will also examine whether a “primary barrier” existed or not in the course of geomorphic development and how the primary barrier enclosed a bay or a drowned valley.
Five coastal lowlands along the Suruga Bay-the Matsuzaki, Kano River, Ukishimagahara, Seishin, and Haibara Lowlands-are studied: all of these lowlands have barriers, and they represent typical coastal lowlands with barriers in Japan.
The presence of “primary barriers” is confirmed in the Ukishimagahara and Haibara Lowlands, where large amounts of sediment are supplied and there is space for sedimentation such as an abrasion platform.
The enclosing of a bay or a drowned valley by a barrier can be detected in the environmental changes, which appear in the fossil foraminiferal assemblages. The initiation of the enclosure process is indicated by a change in the frequency of Ammonia beccarii forma A, which occurs in a lower salinity environment such as the innermost part of a bay. Therefore Ammonia beccarii forma A is considered to be an indicator of a lower salinity environment, and an increase in its frequency in the fossil foraminiferal assemblages is interpreted as the initial stage of enclosure by a barrier. On the other hand, the absence of foraminifera indicates the final stage of enclosure by a barrier, because recent foraminifera live in the sea or in brackish water.
An example of the process of geomorphic enclosure by a barrier at the mouth of a bay or a drowned valley can be summarized as follows:
1. A primary barrier began to form at the time the sea-level was lower, around 8, 000 yr B. P., and migrated landward with the rising of the sea-level.
2. When the rate of sea-level rise decreased, between 7, 000 and 6, 000 yr B. P., the primary barrier reached its innermost position, and thereafter secondary barriers developed in sequence. The primary barrier influenced the sedimentary environment in the bay which was formed inside the barrier, and the bay changed into a lagoon.
3. After the culmination of the Postglacial sea-level rise, the sea-level remained stable or lowered slighty. At this time, the lagoon was completely enclosed by the primary barrier and changed into a swampy area. In the case of the Suruga Bay, the enclosure was completed between 6, 000 and 5, 000 yrB. P.
4. The secondary barriers episodically prograded seaward. Along the Suruga Bay, they completely enclosed the backswamps in two periods, from 5, 000 to 4, 000 yr B. P. and from 3, 000 to 2, 000 yr B.P.

View full abstract