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Home , Bhairab River, Ganges Delta, Landform, Levee

Geographical Review of Japan Vol. 58 (Ser. B), No. 2, 149-164, 1985

Natural and Evolutions

in the Lowland

Masatomo UMITSU*

This paper analizes the characteristics of natural levees and evolution of in the Bengal Lowland. The natural levees in the lowland are classified into four types, such as broad and obscure, broad and distinct, narrow and continuous, and discotinuous types. Narrow and con tinuous natural levees are further sub-classified into those of meandering, dendritic and irregular patterns. Discontinuous natural levees are also sub-classified into an arc-shaped, mottled pattern and natural levees on the bars. Regional distribution of these types of natural levees was considered in relation to the condition of floodings and to the shiftings reconstructed from various documents and maps. Broad and obscure natural levees were formed according to the shiftings of the River in the ancient times. Broad and distinct natural levees were formed along the of the Ganges River by the middle of the eighteenth century. Arc shaped and mottled distributed natural levees were formed by the main flow of the and its distributaries by 1830 AD., during the period when the Brahmaputra River had been flowing along the east of the Madhupur Jungle. After the shiftings of the two , natural levees distributed in a dendritic pattern and a meandering pattern have been formed in the deep flooding area and moderate and shallow flooding areas respectively. Natural levees on the channel bars have been formed along the two rivers and small natural levees of irregularly distributed pattern extended in the under the influence of the .

on the landform evolution of the region. I. Introduction This study is based on the fieldwork carried out during October, 1983 to January, 1984, Natural is one of the most important and indoor works using topographic maps in lanforms in the alluvial lowlands. General scales of 1:50,000 and 1:63,360 (1 mile to 1 features such as distribution, morphology and inch), Irrigation Planning Maps with 5 feet formation of natural levees have been clarified (1.5 meters) contour intervals (1:40,000 scale), by many studies (ALLEN, 1965; RUSSELL, some aerial photos, infrared colored ERTS 1967). However, regional differences in the satellite images in a scale of 1:1 milion and characteristics of natural levees within a single enlarged infrared colored ERTS satellite images alluvial lowland or among several alluvial in a scale of 1:400,000. lowlands have not yet been studied intensive 1y1. The regional characteristics of natural II. Regional setting levees are considered to be deeply related to the formation of alluvial lowlands. Therefore, The Bengal Basin, about 560km long from intensive studies on natural levees with the north to south and 400km wide from east to regional point of view are required for the west, is surrounded by the Tripura and Chit study on formation and landform evolution of tagong in the east, the Rajmahal Hills alluvial lowlands. In this article, the regional in the west, the Sillong and the characteristics of natural levees are studied on Himalaya in the north and faces the region of the Bengal Lowland with the the of Bengal (Figure 1). Ganges Delta which is the greatest delta in The Ganges-Padma and Brahmaputra- the world, and some considerations are made Jamna are the main rivers in the basin. They

* Department of , Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan. 150 M. UMITSU

Figure 1. Map showing the Bengal Basin. 1. Mountains and hills 2. Pleistocene terraces 3. Alluvial fans 4. Older alluvial surface (Tippera surface) 5. Younger alluvial surface 6. Youngest alluvial surface. Contour intervals are three meters.

have often changed their courses in the past. are located on the east of the Rajmahal Hills Now they flow from the west and north respec and on the west of the Tripura Hills, and tively, and join at the central part of the interior ones are located in the central and lowland. Further on, the river which is called northwestern parts of the Bengal Basin. They the Padma joins with the Meghna flowing are called the Madhupur Jungle and Barind, from the northeast, and flows into the Bay of respectively. of the Pleistocene Bengal. terraces in the Bengal Basin are called The landforms of the basin can roughly be Madhupur Clay (ALAM and AURANGZEB, classified into two geomorphological units which 1975; ALAM and KAHN, 1980), which are are Pleistocene terraces and Recent alluvial mainly composed of light brown to reddish lowlands (MORGANand MCZNTIRE, 1959; brown sandy and sandy clay. COLEMAN,1909; UMITSU, 1985). The Pleis Recent alluvial lowlands distribute widely tocene terraces are located in the marginal and over the Bengal Basin. Geomorphologic interior parts of the basin. The marginal ones surface of the lowland is subdivided into three Natural levees in the Bengal Lowland 151 different alluvial surfaces which are older, 3. Narrow and continuous type 4. Discontinuous type younger and youngest surfaces (UMITSU, 1985). The older alluvial surface is called 1. Broad and obscure natural levees by MORGAN and MCINTIRE (1959) as Tippera surface which are slightly higher than Broad and obscure natural levees are recog the younger and youngest alluvial surfaces. nized along the Bhagirathi, Bahairab and some It is located to the east of the other rivers in the northwestern part of the and characterized by the surface with few Ganges Delta (Figure 3). These natural levees systems. Similar surfaces with few develop tonally from northwest to southeast. drainage systems are seen in the opposite side The width and length of the natural levees are of the Meghna near Naria in Faridpur District more than 20km and 100-150km respectively. and in the region to the south of Calcutta City The surface of the natural levees are about of , . These surfaces are 2-3m higher than the backswamps which can be also considered to be correlated to the Tippera recognized as slightly lower zonal depressions. Surf ace2). Younger alluvial surface is broadly Numerous cut-off and abandoned river distributed in the alluvial lowland of the channels are distributed on the surface of the Bengal Basin. The youngest alluvial surface natural levees. Width of these cut-off lakes is located along the courses of the Ganges and abandoned river channels are 0.5-1.5km, Padma and Brahmaputra-Jamna Rivers. and they are wider than those of the present The alluvial lowlands of the central and rivers flowing in the region. Sediments of the southern Bengal Basin, to the south of the natural levees are composed of brownish light Sillong Plateau and Barind, can roughly be grey sandy silt. divided into the of the Ganges Delta, 2. Broad and distinct natural levees Brahmaputra-Jamna and Sylhet Basin. The heights of the lowland are mostly In the central area of the Ganges Delta, below 15m. The altitudes of the central Sylhet broad and distinct natural levees develop along Basin and the southern Ganges Delta are the Bhairab, Madhumati and some other rivers. especially low, which are usually below 5m. And the most remarkable ones develop along Landforms of the alluvial lowlands are com the Bhairab River (Figure 4). They extend northwest to southeast from the southeastern posed of slightly higher landform units such as natural levees, point bars, channel bars, end of the broad and obscure natural levee ridges3), and relatively lower landform zone. The width of them are 3-5km, and the units such as backswamps, marshes, former lengths are about 50-60km long. Boundaries river channels. There is a considerable dif between the natural levees and backswamps ference in their forms and distributions among are fairly sharp and clear, and the relative the regions. Sediments of the alluvial low heights between them are about 2-3m. Surface are mainly composed of non-weathered sediments of the natural levees are composed , silt and clay layers, and peat layers can of brownish light grey sandy silt. be seen in several places. 3. Narrow and continuous natural levees

III. Characteristics of natural levees a. Natural levees extending in a in the Bengal Lowland ing pattern Narrow and continuous natural levees dis Natural levees on the younger and youngest tribute widely in the Sylhet Basin, the northern alluvial surfaces in the central and southern and the eastern parts of the Ganges Delta. Bengal Lowland are studied in this article. In the eastern Sylhet Basin, continuous Various features of natural levees4) can be natural levees bend and extend along the rivers seen in the Bengal Lowland, and they are such as the Surma, Barak and Kusiyara. The classified into the following types (Figure 2). widths of natural levees are about 0.5-1km, 1. Broad and obscure type and the relative heights from the backswamps 2. Broad and distinct type called "", are about 1.5-2.5m. Sediments 152 M. UMITSU

Figure 2. Typical natural levees in the Bengal Lowland . 1. Broad and obscure natural levees 2, Broad and distinct natural levees 3. Narrow and continuous natural levees (a) Natural levees extending in a meandering pattern (b) Natural levees extending in a dendritic pattern (c) Natural levees extending in an irregular pattern 4. Discontinuous natural levees (a) Natural levees distributing in an arc-shaped pattern (b) Natural levees distributing in a mottled pattern (c) Natural levees on the present and former channel bars. Locations of each figure are shown in Figure 8. 1. Natural levees 2, Back 3. Former river channels 4 . Water surfaces 5. Small cliffs of them are composed of brownish light grey tinuous natural levees extend mainly in the silt. backswamp zones of the region, and they ex At Kanailgat to the northeast of Sylhet, the tend along the present small rivers such as the thickness of the natural levee is about Chtra, Nabaganga, Kumar, etc. Widths of 3m, and under which, there are alternative them are 0.8-2.5km, and relative heights from layers of bluish grey silt and fine sand with their backswamps are about 1.5-2.5m. 6 m thick. At the lowest part of the alterna b. Natural levees extending in a dendritic tive layers, there is a veneer of a peat layer. pattern Radio-carbon age of the peat was measured Natural levees extend in a dendritic pattern at 4,180•}120 yBP (GaK-11955). in the central Sylhet Basin, central part of the A similar type of natural levees develops in Ganges Delta, south of the Barind Terrace and the northern and western parts of the Ganges some other small areas. Delta. In this region, there are also some broad In the central Sylhet Basin, there is a region and obscure natural levees. Narrow and con with marshy lands and numerous bogs ex Natural levees in the Bengal Lowland 153

butaries from the main rivers of the region, and extend into the depressions called "heels". The width and length of them are 0.5-1km, 3-5km respectively. Sediments of the natural levees are composed of light grey sandy silt. To the south of the Barind terrace, there is a swampy region which spreads 100km from east to west and 65 km from north to south. In the western part of the region, relatively large natural levees branching in a dendritic pattern extend from the westernmost part of the region near the Ganges River to the north east. The widths of the natural levees are about 2-3km, and it becomes narrower towards the northeast while the number and size of swampy depressions called "" increase towards this direction. Natural levees extend ing in a dendritic pattern can also be found in the regions near Ghatal City, west of Calcutta, and southwest of City, between the Dhaleswari and Padma Rivers. c. Natural levees extending in an irregular

Figure 3. Landform classification map of eastern pattern Kushtia District. Dense and complicated distributing pattern 1. Broad and obscure natural levees 2. Normal natural levees of natural levees develops in the (including broad and distinct natural levees, small and continu and its neighboring regions facing the Bay ous natural levees) 3. Backswamps 4. Swampy lands (including of Bengal. Large rivers and densely distri swampy backswamps and abandoned river channels) 5. Water surfaces. Location of the map is shown in Figure 8. buted small rivers distribute in the region. Natural levees in the region develop along tending for 50km from east to west and 110 these small rivers, and distribute in an irregular km from north to south. The altitude of the pattern. The widths of them are 150-500m. region is mainly below 5m, and the lowest 4. Discontinuous natural levees point of the region is 1.8m high above level. Natural levees in the region develop a. Natural levees distributing in an arc along the rivers such as the Surma, Nakunda, shaped pattern Kusiyara, Kangsa, Saiduli, etc., and show den In the region along the Old Brahmaputra dritic patterns in their distribution. In the River of the western Sylhet Basin, natural rainy season, the natural levees extend into levees distribute in an arc-shaped pattern branches towards the broad water bodies which (Figure 5). They are about 0.5km in width spread out all over the central Sylhet Basin. and 5-8km in length, and they become con The width of the natural levees is 0.5-1km, tinuous and bending frequently as heading and the relative height from the backswamps eastward. The backswamps are also arc or is 1-2.5m. semi-circular in shape. The relative heights In the central part of the Ganges Delta, between the natural levee crests and back swampy region extends into Faridpur, Jessore swamps are about 1.5-3m. Arc or semi and Districts of . It covers circular shaped water areas often distribute a large area stretching over 80km from east to between the natural levees. These natural west and 65km from north to south. Natural levees seem to have both characteristics of the levees extending in a dendritic pattern are normal natural levees and point bars. distributed in and around this swampy region. Similar pattern of natural levees can be seen These natural levees develop along small tri along the Meghna River in the southern part 154 M. UMITSU

Figure 4. Landform classification map of Khulna region . Legend is the same as for Figure 3. The location of this region is shown in Figure 8 .

Figure 5. Landform classification map of the Old Brahmaputra floodplain . Legend is the same as for Figure 3. The location of this region is shown in Figure 8 . of the Sylhet Basin. The Meghna system in terraces, many small natural levees distribute the region, however, shows an anastromatic in a mottled pattern. Some of them distribute pattern. continuously and have a tendency to follow b. Natural levees distributing in a mottled the former river courses. The width and length pattern of these natural levees are narrower than In the Brahmaputra Jamna , 1km, and shorter than 3km, respectively . between the Barind and Madhupur Jungle Backswamps along the western margin of the Natural levees in the Bengal Lowland 155

Madhupur Jungle remain as a swampy zone The sediments of the natural levees are com even in the dry season. The relative height posed of brownish light grey silt or sandy silt, between the natural levees and backswamps but those of the channel bars are composed is about 1.5-3m. of grey medium to course sand. c. Natural levees on the present and former channel bars IV. and river shiftings in the The riverside regions along the Ganges Bengal Lowland Padma and Brahmaputra-Jamna are slightly lower than the surrounding areas. Small 1. Floods in the Bengal Lowland cliffs with a relative height of 1-3m extend Seasonal changes of rainfall and run-off oc along the boundaries between the younger curs remarkably in the drainage basins of the and youngest alluvial surfaces. Landforms of Ganges, Brahmaputra and their . the region along the rivers consist of present At the Harding over the Ganges River, and former channel bars and average monthly in the dry season channels. Most of the channel bars are very is less than 5,000m3/sec and it is about 2,000 large in size and low in relative heights. And m3/sec in April and May, but in the mid on the present and former channel bars, there rainy season it increases to 35,000-40,000m3/ are small natural levees called "chars". They sec. Similar tendency can be observed in the are thought to be formed and extend by flood region along the Brahmaputra-Jamna. Average as the higher of the channel bars. monthly discharge at Bahadurabad during Jan

Figure 6. Flooding in the Bengal Lowland. 1. Mountains and hills 2. Deep flooding 3. Deep flooding (well drained area) 4. Moderate flooding (Poorly drained swampy area) 5. Moderate flooding 6. Shallow flooding (well drained area) 7. Very shallow flooding After JOHNSON(1982), with modification based on the author's field survey and analysis of ERTS satellite image. 156 M. UMITSU nary to March is less than 5,000m3/sec, and it change into a swampy region. exceeds 40,000m3/sec in July and August. In the region between the Madhupur Jungle Owing to the remarkable increase of the and , there is also a poorly drained river discharge, immence floods prevail over region. The Padma the Meghna at the most part of the Bengal Lowland in the a little downstream from the region, and the rainy season. And the flooding conditions in broad water body connects to that of the the Bengal Lowland show fairly regional dif Meghna River, which is the lower reaches of ferences (Figure 6). the flooding area in the central Sylhet Basin. The most remarkable innundation occurs in Therefore, the innundation in the region be the central Sylhet Basin, where the depth of tween the Madhupur Jungle and Padma River water exceeds 0m in maximum, and deep is due to the damming up of the Ganges and water spread over 50km from east to west and Brahmaputra River's water by the backwater 110km from north to south in the rainy from the Meghna River. The maximum depth season. MORGANand MCINTIRE(1959) pointed of the water in the region exceeds 3m, and out that the region is tectonically subsiding. most of the water will drain off in the early This can be supported by the westward tilting dry season. of the Madhupur Jungle terrace and the north Deep water in the rainy season also spreads ward inclination of the northern tips of the over the youngest alluvial surfaces along the Tripura Hills. Ganges-Padma and Brahmaputra-Jamna Riv In the regions between the Barind and the ers. In some places, the depth of the water Ganges left , central part of the Ganges exceeds 3m, and the main flow of the region delta and northwest of Calcutta City, there is fairly rapid in the rainy season. Then the are also broad swampy regions, where exten depth of the water in the region decreases sive flood water spread out in the rainy season. rapidly in the early dry season. The depths of the water are not so deep as in Flooding in the rainy season is not so serious the central Sylhet Basin, but still often exceed in the Brahmmaputra-Jamna floodplain, the 2m. Old Brahmaputra floodplain and the north The of the central Ganges Delta western part of the Ganges Delta. The depths is also pointed out in former studies. Sub of the water in the regions are mostly shallower sidence of the region is deduced from the fact than 2m, and it drains rapidly in the dry that many tree stumps were found in the season. Considerable areas in the northwestern recent sediments at depths varing from G-100 part of the Ganges Delta, which is called the feet (MORGANand MCINTIRE,1959; LATIFUL Moribund Delta by BAGCHI(1944), are free BARIed., 1978). MORGANand MCINTIRE(1959) from floods. pointed out that the cause of subsidence in the The region in and around the Sundarbans deltaic region is owing mainly to the compac facing the is very low and , tion of Recent sediments and structural down but innundation by floods is negligible. This warping. is because that the region is facing the sea and On the other hand, MORGANand MCINTIRE dense small rivers develop throughout the (1959) pointed out that there is a faulting in region, and therefore the floods drain easily the northern part of the Barind terrace. and rapidly. Almost all of the region is free Another faulting in the southeastern part of from floods or very shallow flooding area. the Barind can also be recognized.5) These However, the region is extremely weak against f aultings accompany the southwestward tilt the storm tide caused by cyclones. Broad area ing of the southeastern part of the Barind. will be washed away by a single storm tide. Such tilting seems to cause the subsidence to The height of the tide sometimes exceeds 4m the south of the Barind. Besides this tectonic above the average tide level, but the tide movement, the southern margin of the swampy ebbs rapidly. region has been risen by the of the Ganges River. This makes the region 2. River shiftings in the Bengal Lowland between the Barind and the Ganges River Rivers flowing in the Bengal Lowland have Natural levees in the Bengal Lowland 157

Figure 7. River shiftings in the Bengal Lowland. 1. Mountains and Hills 2. Pleistocene terraces 3. Alluvial fans 4. Flood and delta 5. Before the year 6. After the year often changed their courses. Especially in the RIZVI (1955) mentioned that the Bhagirathi Ganges Delta, the Ganges River with several channel was considered to be an old course of distributaries such as Bhagirathi, Bhairab, the Ganges until the sixteenth century, but Garai, Anal Khan, etc. has changed its course it was not possible to deny that the Padma into these distributaries one after another had once existed as a main course in the (Figure 7). Christian era. The oldest course of the main flow of the However, MUKERJEE(1953) concluded from Ganges River is considered to be the Bhagirathi, the account of the Moorish traveller, Ibn the westernmost of the Ganges BATTUTA(1328-54), that in the fourteenth (BAGCHI,1944; RIZVI,1955). A number of dis century the of the Ganges and tributaries between the Bhagirathi and the Brahmaputra was very near the of Chit Padma, the easternmost distributary of the tagong. This implies that the Ganges River Ganges, became successively the main course flowed into or around the Padma River at of the Ganges. BAGCHI(1944) pointed out that that time. Furthermore, in a French map "Carte du voyage tres the Padma River course has already existed -curieux et fort remarqu since 300 B. C. based on the MEGASTHENES'S, able" published in 1252 A. D.,6) a river which PERIPLUS'Sand PTOLEMY'Saccounts of India. seems to be the Ganges was flowing along the 158 M. UMITSU

course similar to that of the Padma. Maps in the piracy in the upper reaches of the Brah the middle sixteenth century shown by CHO maputra, and the sudden diversion of the UDRY(1976) also indicate the rivers of the Bhagirathi Tista River water from the Ganges to the and Padma. And in Bakerganj District of Brahmaputra in 1787 was the final action Bangladesh, there are some ruins of old mosque that triggered the diversion into the Janai constructed in the middle of the fifteenth (Jamna) channel west of the Madhupur Jungle. century (RASHIDed., 1980). These are located MORGANand MCINTIRE(1959) explained the on the southeastern part of the Ganges Delta, shifting of the Brahmaputra by the tilting and the right bank of the Meghna confluenced movement of the Madhupur Jungle in addition by the Padma. This means that the to the change of the course of the Tista River. around the lowest reaches of the Padma was OYA(1977) also supported the thought of MORGAN already formed in the middle of the fifteenth and MCINTIRE(1959). century. All these facts indicate that the main waters of the Ganges River have already been V. Some considerations on the forma flowing in the course of the Padma before the tion of natural levees sixteenth century. After the seventeenth century, the Anal Various types of natural levees develop in Khan had already been an important river in the Bengal Lowland (Figure 8). The largest 1624 A. D., and Van den BROUKE'Smap of type of natural levees develop in the north 1660 A. D. also shows it as the main river of western part of the Ganges Delta. As the the Ganges. In the beginning of the nine magnitude of this type has no comparison teenth century, the Garai-Madhumati River with that of the present rivers flowing there, suddenly grew in size and became an important it is not reasonable to consider that these distributary of the Ganges waters. The de natural levees have been formed by the pre velopment of the Garai was due to the diver sently flowing rivers in the region. In general, sion of the Brahmaputra through the Jamna large natural levees are formed along large channel meeting the Ganges at Goalund, there rivers, and obscure natural levees seem to be by the Ganges flood headed up to a certain formed in older age. The main waters of the extent and it was forced to find some other Ganges River flowed through this region in outlets (RIzvi, 1955). the ancient times. And it is acceptable to On the other hand, in the RENNEL'Smap consider that some large natural levees were published in 1779, the course of the Brah formed along the former main courses of the maputra River was shown to the east of the river. Then, the large and obscure natural Madhupur Jungle. The diversion of the Brah levees in the northwestern Ganges Delta are maputra into the channel of the Janai (Jamna) considered to be formed along the former had occurred by early 1770's and the shiftings courses of the Ganges River. of the river into the present channel to the The broad and distinct natural levees along west of the Madhupur Jungle had completed the Bhairab, Madhumati and some other in 1830 (RIZVI, 1955; MORGANand MCINTIRE, rivers develop in the region extending to the 1959). southeastern part of the broad and obscure MORGANand MCINTIRE(1959) also explained natural levees. As they develop along the the river shifting of the Tista River. It flowed rivers and the boundaries between the natural along the course of the Dhepa-Purnabhaba levees and backswamps are distinct, the age River, west of the Barind, and discharged into of formation of the natural levees seem to be the Ganges in the RENNEL'Stime. And it has not so old. As for the flow of the Bhairab changed its course to the present one flowing River, there is a record that the upper channel into the Brahmaputra in 1787 during a single of the Bhairab, which had earlier deteriorated, flood. was forced to open by a heavy flood in 1874, LATOUCHE(1910) suggested that the Brah and took waters from the Ganges (RIZVI, maputra diversion resulted directly from a 1955). And on the flow of the Madhumati, major increase in discharge of the river by there is also a record that the Garai-Madhumati Natural levees in the Bengal Lowland 159

Figure 8. Distributions of the various types of natural levees. 1. Mountains, hills and Pleistocene terraces 2. Older alluvial surfaces 3. Broad and obscure natural levees 4. Broad and distinct natural levees 5. Continuously meandering natural levees 6. Dendritic extending natural levees 7. Natural levees extending in an irregular pattern 8. Arc-shaped natural levees 9. Natural levees distributed in a mottled pattern 10. Natural levees on the channel bars 11. Alluvial fans 12. Beach ridges 13. Locations of Figures 3-5 14. Locations of maps in Figure 2 15. Location of Kanailgat 160 M. UMITSU

River suddenly grew in size and became an of the Madhupur Jungle. important distributary of the Ganges in the On the other hand, natural levees on the beginning of the nineteenth century . How Brahmaputra-Jamna flood plain show a mot ever, in the RENNEL'smap (1977), the Bhairab , tled distributed pattern which is quite different Madhumati and some other rivers which ac from the arc-shaped natural levees along the company broad and distinct natural levees Old Brahmaputra River and the landforms of already existed along the present courses. the youngest alluvial surface along the present Furthermore, Khulna City which is situated on Brahmaputra-Jamna River. And it will not the natural levee of the Bhairab River, has be reasonable that these natural levees were already existed there in the fifteenth century formed by the main flow of the Brahmaputra (LATIFULBARI ed., 1978). Therefore, it is con River. From the characteristics of relatively sidered that the broad and distinct natural small and discontinuous mottled distribution levees were already formed by the middle of of the natural levees, it is considered that these the eighteenth century, and the natural levees natural levees were formed by relatively small along the Bhairab River was formed by the rivers and that they are not so young. Then, fifteenth century. They, however, were not the natural levees of mottled distribution on formed in so earlier than that time. the Brahmaputra-Jamna flood plain are There are arc-shaped natural levees in the thought to have been formed by the small region around the north of Mymensing City rivers like the Old Jamna (lanai) River in the and the west of the Madhupur Jungle. These past when the main flow of the Brahmaputra natural levees are quite different in shape from was flowing in the east of the Madhupur those along the small rivers around the region . Jungle terrace. These arc-shaped natural levees develop along Continuous small natural levees extend along the Old Brahmaputra River from the west to the rivers in most parts of the Sylhet Basin and the east. The Old Brahmaputra River is a Ganges Delta. They are obviously formed by relic of the former Brahmaputra River, and the present rivers flowing in the regions. she is now relatively narrow. Before 1830 A.D., Natural levees showing the dendritic pattern however, a large and broad flow of the main develop in the regions of the central Sylhet Brahmaputra flowed in the region along the Basin, central Ganges Delta near Khulna , present course of the Old Brahmaputra River. south of the Barind terrace, south of the Therefore, the arc-shaped natural levees along Madhupur Jungle terrace and so on. All these the Old Brahmaputra River are thought to regions are covered by the considerable depth have been formed by the main flow of the of the waters in the rainy season. The drain former Brahmaputra River by 1830 A. D. age condition of the water in the regions is Similar natural levees with the shape of arc very bad, and broad flooding areas have ex distribute along the present course of the isted for a long period. Under the condition Meghna River from the south of the Sylhet like this, tips of the river mouths extend into Basin to the confluence with the Padma. This the broad water areas with sedimentary loads , suggests that the sedimentary condition of the and the mouths of rivers bifurcate one after region along the Old Brahmaputra River by another. Then the natural levees showing 1830 A. D. seems to be similar to that of the a dendritic pattern is formed in the regions. region along the present Meghna River . The At Kanailgat along the Surma River in the sedimentary condition is considered to be less Sylhet Basin, sediments under the natural loaded than that of the present Brahmaputra levee deposits are composed of bluish grey Jamna River. And this is concordant with sand and silt alternative layers, which are the increase of loads that was caused by the considered to be the sediments of shallow shifting of the Tista River. The shifting of lakes. And the age of the lowest part of the the Tista River with a large quantity of alternative layers is measured at 4,180•}102 sedimentary loads in 1787 is supposed to be yBP. These characteristics of the sediments one of the reasons causing the shifting of the and their age indicate that the region has been Brahmaputra River from the east to the west in such a condition of a shallow influenced Natural levees in the Bengal Lowland 161

by annual floods since ca 4,000 yBP, and it present. The former was formed in accordance has changed into a flood plain, recently. with the shif tings of the Ganges and Brah The continuously meandering natural levees maputra Rivers. They are broad and obscure extend in a wide region of the Bengal Lowland, natural levees, broad and distinct natural where it is characterized by the normal flood levees, discontinuous natural levees distri ing condition. Furthermore, the continuous buting in an arc-shaped and in a mottled meandering natural levees also can be develop patterns. The latter is the small and con ed in the northwestern part of the Ganges tinuous natural levees and natural levees on delta, where the flooding condition is relative the channel bars. These natural levees formed ly shallow. by the present rivers closely relate with the The riverside lowlands along the Ganges difference of flooding conditions in the Bengal Padma and Brahmaputra-Jamna are slightly Lowland. Narrow and continuous natural lower than the adjacent . Streams levees extending in a meandering pattern of the both rivers indicate a braded pattern in develop in the regions of moderate and relative the most part of the region, and natural levees ly shallow floodings. The natural levees ex are found on the broad channel bars and river tending in a dendritic pattern can be found in side lowlands. The natural levees distribute the regions of deep flooding or moderate flood irregularly, and shallow depressions often sur ings, where the drainage condition is very round them. In the rainy season, most of the poor and broad flooding areas exist for a long region is covered by deep water of floods, and period of time. The natural levees on the the water drains off rapidly in the early dry channel bars and adjacent floodplains are season. Most of the natural levees emerge at formed along the present flows of the Ganges that time and the depressions surrounding Padma and Brahmaputra-Jamna Rivers. Most them changed into the part of the braided of them are under floods in the rainy season streams. Therefore, these natural levees are and emerge in the early dry season. considered to be a kind of temporal channel The broad and obscure natural levees were bars on the broadly distributed channel bars formed in the period when the main flow of in the dry season. the Ganges existed between the Bhagirathi Natural levees showing the irregulaly dis and Padma. The broad and distinct natural tributing pattern in and around the Sundar levees are considered to be formed after the bans extend along the present rivers in the formations of broad and obscure natural levees, region. The drainage system of the region and they were already formed by the middle indicates the typical pattern of the tidal delta, of eighteenth century. Arc-shaped natural and it is considered to be closely related with levees along the Old Brahmaputra River were the tidal agency. And the flooding condition formed before 1830 A. D. by the main flow in the region is very shallow. Therefore, the of the former Brahmaputra River. And small-scale natural levees in the region are natural levees distributing in a mottled pattern considered to be formed under the condition on the Brahmaputra-Jamna floodplain were that the sediments brought from the upper formed by the small rivers like the Old Jamna reaches of the rivers deposited along the river (lanai) River in the past when the main flow side according to the tidal effects. And an of the Brahmaputra was flowing east of the abundant growth of the such as Madhupur Jungle. along the riversides also seems to be After the shiftings of the Ganges and Brahm effective for the of the sediments. aputra Rivers to the present courses, small and continuous natural levees and natural levees VI. Conclusion on the channel bars along the present rivers have been formed in the Bengal Lowland. The Various types of natural levees develop in formations of the natural levees in the Bengal the Bengal Lowland. They are classified into Lowland are shown in the paleogeographic map the natural levees formed by the rivers in the of Figure 9. past and those formed by the rivers in the 162 M. UMITSU

Figure 9. Geomorphological evolution of the Bengal Lowland.

1. Mountains, hills and Pleistocene terraces 2. Older alluvial surfaces 3. Broad and obscure natural levees 4 . Broad and distinct natural levees 5. Continuously meandering natural levees 6. Dendritic extending natural levees 7. Natural levees extending in an irregular pattern 8. Arc-shaped natural levees 9. Natural levees distributed in a mottled pattern 10 . Natural levees on the channel bars 11. Alluvial fans 12. Beach ridges

MIAH, geomorphologist and associate professor at Acknowledgments Dhaka University, Dr. ARM. Fariduddin BHUYAN, principal scientific officer at SPARRSO (Space Re The author wishes to express his sincere gratitude search and Organization) and Mr. to Dr. Fazlul HAQ, chairman of the Department of F. KHARIM,geologist at of Bangla graphy in Dhaka University, Dr. M. Maniruzzaman desh. In particular, Dr. MIAH kindly adviced and Natural levees in the Bengal Lowland 163 helped the author on his survey in Bangladesh. The Ministry of Petroleum and Mineral Resourses, author also wishes to express his sincere thanks to Govt. of People's Republic of Bangladesh, 57-60. Mr. SHIRAI,lecturer at Language Laboratory of Dhaka ALLEN, J. R. L. (1965): A review of the origin and University, and Mrs. SHIRAI who gave the author characteristics of Recent alluvial sediments. Sedi various conveniences during his stay in Bangladesh. mentology, 5, 91-191. (Received June 10, 1985) BAGCHI,K. (1944): The Ganges Delta. University of (Accepted August 7, 1985) Calcutta. Press, 157p. COLEMAN,J. M. (1969): Brahmaputra River: Channel Notes processes and sedimentation. Sedimentary Geology, 3, 129-239. 1) On the landforms of the Bengal Basin, OYA JOHNSON,B. L. C. (1982): Bangladesh. Second edition, (1976) made a geomorphological landform classi Heinemann Educational Books, London, 133p. fication map in a scale of 1 to 1 million using LATIFULBARI, ed. (1978): Bangladesh District Gazet ERTS satellite images, but natural levees are teers: Khulna. Bangladesh Government Press, not classified in detail. Dacca, 452p. 2) The area, south of Calcutta City, is located in LAToUCH,T. H. D. (1910): Relics of the great Ice Age the inner part bordered by the rows of beach in the plains of northern India. Reprinted (1919) in ridges. Report on the Hoogly River and its headwaters, 3) Near the mouth of Hoogly River, south of Cal Calcutta. The Bengal Secretariat Book Depot, 1, cutta City, remarkable beach ridges extend 21-22. along the coastline of the Bay of Bengal. The MORGAN,J. P. and MCINTIRE,W. G. (1959): Quaternary beach ridges can be classified into four rows, geology of the Bengal Basin, East and and the second inner row is mostly developed. India. Bulletin of Geological Society of America, 70, 4) As it is difficult to separate the natural levees 319-342. from point bars in the Bengal Lowland in detail, CHOUDHURY,M. I. (1976): The maps of Bangladesh both of them are classified together as natural during the Muslem period (1202 A. D. to 1757 A. D.). levees in a broad sense. The Journal of the Institute of Bangladesh Studies, 5) There are remarkable differences of the bound 1, 145-154. aries between the Barind terrace and surround MUKERJEE,R. (1953): Ibn Battuta. Routledge and ing alluvial lowlands boundaring the south Kegan Paul Lt., London, 267p. easternmost part of the Barind. The boundary OYA, M. (1976): Geomorphological map of the R. Brah of the northeastern edge is sharp and straight, maputra-Jamna and R. Ganges Plain at the scale of but that of the southern edge is uneven 1:1,000,000. Kokudo Chizu Co., Tokyo. and indistinct. The buried Pleistocene terrace OYA, M. (1977): Applied geomorphological study on bordering the southern margin of the Barind the selection of the proposed bridge-site along the terrace can be seen under the neighboring allu Jamna River in Bangladesh. National Geographer vial lowland. Rivers on the southeastern Barind (Allahabad), 12 (2), 101-113. terrace also flow from northeast to southwest. RASHID, M.H. (1980): Bangladesh District Gazetteers: Therefore, these facts suggest a southeastward Baherganj. Bangladesh Government Press, Dacca, tilting of the Block with a northwest-southeast 416p. faulting along the northern margin of the Barind RIZVI, A. (1955): Comparative physiography of the of the region. lower Ganges and lower valleys. Doc 6) The map is shown in CHOUDHURY(1976). toral dissertation, State University, 261p. References RUSSELL,R. J. (1967): River and delta morphology. Louisiana State University Coastal Studies Series, ALAM,M. K. and AURANGZEB,M. M. (1975): Geological No. 20, 55p. environment for greater Dacca City. Conference UMITSU .M. (1985): Regional characteristics of the issue of the Bangladesh Geological Society, 34-36. landforms in the Bengal Lowland. Studies in ALAM,M. K. and KHAN, M. R. (1980): Madhupur clay socio-cultural change in rural villages in Bangladesh, and its probable scope of economic uses. In Petro No. 1, 1-44, Institute for the Study of Languages leum and mineral resourses of Bangladesh. Conference and Cultures of and . issue of seminar and exhibition organized by the 164 M. UMITSU

ベンガル低地の自然堤防 と地形変化

海 津 正 倫*

ガ ン ジ ス川 ・ブ ラ マ プ トラ川下 流 に ひろ が る ベ ンガル され, 18世 紀 中 頃 まで に す で に 存 在 してい た と考 え られ 低 地 の 自然 堤 防 を分 類 し,そ れ らの 形 成 環 境,形 成 時 期 る。 一 方,弧 状 お よび斑 状 に 分 布 す る 自然 堤 防 は,ブ ラ お よび 形 成過 程 に 関 す る考 察 を お こな った 。 マ プ トラ川 が マ ドフ プー ル ジ ャ ン グル の東 側 を流 れ て い 本 地 域 の 自然 堤 防 は,大 規 模 に 発 達 す るが 不 明 瞭 な も た1830年 以 前 に,そ れ ぞれ ブ ラマ プ トラ川本 流 お よび 支 の,大 規 模 か っ 明瞭 な もの,連 続 的 に分 布 す る もの,不 流 に よ って形 成 され た と考 え られ る。 ガ ンジ ス川 お よび 連 続 に分 布 す る も の の4種 類 に分 類 す る こ とが で きる 。 ブ ラマ プ トラ川が 現 在 の河 道 を流 れ る よう に な っ てか ら これ ら の うち,連 続 的 に分 布す る も の は さ らに,顕 著 に は,洪 水 時 の水 深 が 深 く長期 間 湛 水 す る地 域 で は 樹 枝 状 蛇 行 す る も の,樹 枝 状 に分 岐 す る も の,小 規 模 に不 規 則 の 自然 堤 防 が,湛 水 深 が あ ま り深 くない か 浅 い 地 域 で は に 分 布 す る も の に細 分 類 され,不 連 続 な も の も,弧 状 に 蛇 行 す る 自然 堤 防 が 発 達 す る。 現 在 の 河 道 沿 い の 地帯 で 分 布 す る もの,斑 状 に分 布 す る も の,河 道 お よび 河 岸 に は,新 旧 の 河 道 州 上 に 自然堤 防 が不 規則 に 分 布 し,ベ ン み られ る河 道 州 上 に 発 達 す る もの に 細 分 類 され る。 ガル 湾 に 沿 った 潮 汐 の 影 響 を 受 け る 地 域 で は非 常 に密 度 大 規 模 で 不 明瞭 な 自然 堤 防 は,紀 元 前300年 頃 まで に の 高 い 水 路 に 沿 って 小規 模 な 自然堤 防 が連 続 的 に分 布 す お け る ガ ンジ ス川 の 河 道 変 遷 に 伴 って,大 規 模 で 明 瞭 な る。 自然 堤 防 は,ガ ンジ ス川 の 派川 に よ って それ 以 後 に形 成

* 〒464名 古屋市千種区不老町 名古屋大学文学部地理学教室