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Wetlands along the Dhansiri River channel, Assam

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.

Conclusion :
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.

Acknowledgements:
The authors are grateful to the Director, Regional Research Laboratory (CSIR), Jorhat, Assam, India, for his kind permission to publish the present study.

References :
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