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Wetlands along the Dhansiri River channel, Assam
M. K. Dutta
Regional Research Laboratory(CSIR)
Jorhat 785006, Assam.
Regional Research Laboratory(CSIR)
Jorhat 785006, Assam.
The sequential change in numbers and areas of wetlands (Bils) along the stretch of the Dhansiri River channel have been studied using available topographic maps of the Survey of India (1914,1975) and Indian Remote Sensing satellite imagery (1990,1995,2000). The total number of swamps with dominant oxbow type as evident from the study was 75 in 1914, 127 in 1975, 111 in 1990, 114 in 1995 and 122 in 2000. Moreover, total areas covered by the wetlands as observed was 10.24km2 in 1914, 13.61km2 in 1975 and 13.23km2 in 1990, 11.26km2 in 1995 and 12.56km2 in 2000. Morphological adjustment of stream channels over a short period of time and location of the basin in a tectonically active region have played dominant roles in changing the nature of swampy lands within the basin. It is expected that this exploratory investigation will help to understand the geomorphic development of the region intimately connected with the stabilization of landforms and different wetland management approaches.
The global extent of wetlands is estimated to be from 7 to 8 millions km2 and, compared to the other ecosystems, wetlands are an extremely productive part of the landscape with average annual production above 1000 g cm-2 yr. -1 in terms of nutrient recycling and storage, plant and animal harvest, and species conservation ( Dugan, 1993: Mitsch et al. 1994). To functions necessary for biological interactions between the elements of soil, water, plants and animals together with their biological diversity and the cultural values, the wetlands are marked invaluable to the people all over the world.
The low-lying areas of the floodplain beyond the natural levee are known as back swamps or flood basins over which the finer materials are deposited during overbank floods. These low-lying areas are naturally waterlogged and marshy areas, serving as regulators of hydrologic regimes, maintaining environmental quality, and resources of diverse aquatic fauna and flora. These areas in local parlance are known as ‘Jalah’, ‘Doloni’, ‘Pitoni’, ‘Hola’, ‘Doba’ etc. The reclamation of these disconnected flood basins for cultivation has brought about a general change in their morphology, however, these can still be identified on toposheets and satellite imageries. The perennially waterlogged areas are termed as ‘Bils’ in Assam. These water bodies are of different sizes and shapes and generally connected to adjacent river systems. There are approximately 1392 bils in the Brahmaputra Valley, Assam, of which 423(34.40%) are registered fisheries while the remaining 969(69.6%) are unregistered (Deka and Goswami, 1992). Bils originated as a consequence of many naturally interrelated processes and factors. However, the development of bils in this region is strongly related to the geomorphologic development of the river basin, related neotectonic activity and other allogenic processes.
The rationale behind this type of study is to generate proper prior attention for the maintenance of natural wetlands in relation to different wetland management projects and their expected positive consequences towards development of rural economy. Wetlands are also beneficial for partial flood storage because runoff is temporarily stored. By storing rainwater and releasing runoff, wetlands can diminish the destructive onslaught of floods downstream. Preserving natural storage can also help to avoid the costly construction of dams and reservoirs. Recharge in wetlands in turn supports more stable biological diversity. Floodplain wetlands in NE region of India are under extreme pressure from farm expansion, water control and human habitation. Apart from the numerous aquatic flora and fauna, wetlands in this region are also the winter home for large number of birds and species.
Most of the swampy lands in Assam are formed as ox-bow lakes or abandoned channel scattered all over the active flood plains of the mighty Brahmaputra River system. A few of these water bodies, however, owe their origin to depressions caused as a result of earthquakes. A swamp or bil is a complex natural system whose origin and development depends on a variety of interacting natural as well as biotic/anthropogenic features. Any attempt to understand such a system will require consideration of several aspects related to various geological, hydrological, biological and socio-ecological parameters.
The rivers of the NE region of India manifest frequent changes in morphometric behaviour with resultant changes in associated fluviosedimentological attributes. It is therefore felt that the characterization and quantification of different types of swamp along the stretch of the river Dhansiri within a specific geomorphic setup may throw an insight into their origin and development. The study area is included between 93030/ E to 940 0/ E longitudes and 260 00/ N to 260 45/N latitudes and covers a total length of about 185km (Fig.1). The total length of the channel has been divided into sectors by 30km each so that a detailed evaluation could be made to have a clear idea about its characteristics and geomorphologic evolution. The configuration of the bils within each reach has been investigated to evaluate their significance (Deka and Goswami, 1992: Sharma and Goswami, 1993). Five different types of swamp are identified and characterised (Figure 2) as i) compact(C), ii) linear(L), iii) irregular(I), (iv) discrete(D) and (v) ox-bow(O) and the respective populations of each were counted. The areas of the different bils were measured and the generated data used for the present study. A number of field checks have also been conducted to understand the characteristics and comparative evaluation of the data resource as evident from toposheets and imageries.
Figur 1 : Location map of the study area.
Brinson et al. (1994) classified wetlands into four broad geomorphic groups: riverine, depressional, fringe (coastal) and extensive peat lands and known as Brinson's Hydrogeomorphic Classification (HGM). Wetlands are also classified according to the source of water (precipitation, overland flow and ground water) and hydrodynamics (the strength and direction of flow). In the present study the wetlands or swampy lands are classified following Deka and Goswami (1992) and Sharma and Goswami (1993) in relation to localised basinal characteristics, flow behaviour, soil type, and geomorphologic attributes.
Figure 2: Different types of wetlands (Swamps)
The present approach employs Survey of India (SOI) toposheets (1914 and 1975) and Indian Remote Sensing (IRS) satellite imagery (1990, 1995 and 2000) to investigate spatial changes over available period of time. Such information is considered valuable in providing information for a period of 87years, which is often beyond the scope of empirical observation. The Survey of India toposheets of 1975 (Scale 1:50,000) were scanned, georeferenced, mosaiced and used to prepare the base map. The Survey of India toposheets from 1910-1914 (Scale 1:63,360) were scanned, registered and resampled. The IRS black and white (B/W) imageries on band four (wave length 0.77 to 0.86µm, with spatial resolution 36.25m, Linear Imaging and Self Scanning II) at 1:50,000 scale were also scanned, registered and resampled to keep the uniformity in data set. The satellite imageries and SOI toposheets from 1910-1914 were registered to the base map using a set of Ground Control Points (GCPs) in ERDAS IMAGINE 8.5 software. Thematic maps of different periods were prepared on 1:50.000 scale and were integrated using Arc view GIS. The meanders were then characterized, quantified and natures of movements were measured to generate database for the present exploratory study.
Results and discussion:
The different types of wetlands are not uniformly distributed through out the Dhansiri River basin. The number of wetlands and their corresponding areas for five different period of study are projected through Figures 3 & 4. The pre 1914 period as evidenced from the Survey of India (SOI) toposheets exhibited dominant association of linear
Figure 3: Development of Swamps in different periods
variety of swamps (26.67%) and represented mainly by the paleochannels of the earlier river. The association of different varieties of swamps can be represented as L(26.67%) >C(24%)>O(22.67%)>D(20.0%)>I(6,67%). Out of six sectors the Sector 1 represents highest development of wetlands with equal proportion (36.36%) of compact and linear varieties. However, on closer observation in terms of areas covered by the wetlands, in all sectors other than Sector 4 within the studied stretch abundant presence of oxbow type swamps was clearly observed. The abundance of oxbow type swamps has been clearly
Figure 3: Spatial distribution of Swamps in different periods
attributed their origin from the earlier meandered channels. The maximum number and land area covered by the oxbow type swamp were observed in the Sector 3. The SOI toposheets of 1975 has demonstrated almost an opposite picture of 1914 observations. The distribution of total number of wetlands can be represented as O(45.67%)>L(25.98%)>C(18.11%)>I (7.87%) >D(2.36%). In the 1975 geomorphic map maximum number (47) of wetland was observed in the Sector 3 however, maximum land area (5.07km2) of wetlands was observed in the Sector 2. The maximum developed sector i.e., Sector 3 can be represented as O(46.80%)>C(31.91%)>L(8.51 %)>D(6.38%) = I(6.38%). The effect of the ‘Great Assam Earthquake’ of 1950 with magnitude 8.6 on Richter scale and the attendant historic flood probably have played a key role in changing the morphology of the area. Before 1950, there was a balance between sediment supply and transportation. After 1950, this balance was disrupted as the great earthquake caused generation of extensive pile of sediments (Goswami et al., 1999) and initiated modification and adjustment of channel form. The resultant change in hydrologic regimen has contributed to significant changes in the geomorphology of the region. Migration and abandonment of channels over a short period of time have resulted in the generation of many low-lying areas adjacent to the river system. The channel experienced a differential rate of geomorphological change as the geology and soil type along the channel is not identical. The extensive development of swamps as represented by 1975 Survey of India toposheets, compared to 1914 data set can be explained as the intense effect of neotectonics on the nature of the basin. Pascoe(1964) and Geological Survey of India (1977) advocate, it as due to gradual subsidence of the whole or part of the basin, while others advocate that there is both submergence as well as uplift but with dominance of submergence (Prasad et al. 1981). Many workers (Pascoe, 1964: GSI, 1977: Goswami et al., 1999: Sarma, 1999) have argued that the valley floor is subsiding due to tectonic movement. As the area is a part of seismically active NE-India situated between the Himalayan and the Arakan-Yoma mountain arches the reasoning seems to be plausible. Subsidence thus helps to transform every former channel into a low-lying area (G.S.I.1977). However, the activity is not uniform throughout the basin and as a result abandoned channels become isolated into cut-off meanders or ox-bow lakes. Moreover, construction of antiflood and some other anthropogenic measures also helps to create many big low lying areas adjacent to active channels. The generation of linear and ox-bow lakes is generally related with migration and morphometric adjustment of stream channels at different level of flow conditions. It is difficult to answer why and how the discrete nature of swamps connected by small segments of channels and irregular low lying structures are generated. However, it can be presumed that the existing lithology of the area, channel network, and associated autogenic processes might have played the key role in the geomorphologic changes of swamps.
Almost similar observation has also been observed in the geomorphic map of 1990. It has clearly been attributed from the study that during the period 1975 to 1990 no significant geomorphic changes took place within the stretch under study. Maximum land area (5.21km2) covered by the wetlands was observed in the Sector 2 and was followed by Sectors 3(3.49km2) and 1(2.76km2). The distribution of wetlands during this period can be represented as O(35.14%)>L(34.23%)>C(27.03%)>I(3.60%). The rate of geomorphological change took place during the period 1990 to 1995 was appeared to be somewhat different than the earlier periods. The total number of wetlands during this period can be represented as L(38.60%)>O(35.97%)>C(22.81%)>D(1.75%)>I(0.88%). Sector 2 has represented association of both maximum number (35) and land area (3.763km2) of swamps within the studied stretch for the present period. The dominant linear nature of the swamps testifies to the transient nature of most of the former stream channels of the basin, which on subsidence generate swamps. The period during 1995 to 2000 witnessed a similar picture that of 1975 with dominance of oxbow type wetlands. Maximum number (38) of swamps was observed in the Sector 3. The distribution of total number of wetlands can be represented as O(54.10%)>L(29.51%)>C(13.93%)>I(2.46%). However, maximum land area covered by the wetlands (4.74km2) was observed to be associated with the Sector 2 with dominant oxbow type (3.75km2) wetland.
Swampy land or wetlands have represented potentially very rich areas with geomorphologic and ecologic significance. These low lying areas act as storage basins during flood, reducing the flood impact and minimizing the potential for erosion (Deka and Goswami, 1992). These wetlands are also helpful in maintaining the rich bio-diversity of the region and can be used as an important eco-potential resource.
The geomorphic, tectonic and hydrologic behaviour of the Dhansiri River basin is intimately connected with the morphologic features and genesis of the structures. Morphological adjustment of stream channels over a short period of time and location of the basin in a tectonically active region have played dominant roles in changing the nature of swampy lands within the basin. Land use pattern have also influence the development of many wetland areas. Significant changes on the low-lying areas have been observed during the period 1914 to 1975 in the studied stretch of the river.
The authors are grateful to the Director, Regional Research Laboratory (CSIR), Jorhat, Assam, India, for his kind permission to publish the present study.
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