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Wetlands along the Dhansiri River channel, Assam

M. K. Dutta
Regional Research Laboratory(CSIR)
Jorhat 785006, Assam.

P. Kotoky
Regional Research Laboratory(CSIR)
Jorhat 785006, Assam.

Abstract :
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.

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