Paper presented in the Annual Meeting of Association of American Geographers, Las Vegas, USA, March 2009 ABSTRACT

Transcription

1 Paper presented in the Annual Meeting of Association of American Geographers, Las Vegas, USA, March 2009 ABSTRACT CHANGING GEOMORPHOLOGY OF THE KOSI RIVER SYSTEM IN THE INDIAN SUBCONTINENT Nupur Bose, Department of Geography, A.N. College, Patna, India. Ashok Kumar Ghosh, Dept. of Environment and Water Management, A.N.College, Patna, India, Ajay Govind Bhatt, Dept. of Environment and Water Management, A.N.College, Patna, India The eastern state of Bihar in India is a flood-prone fluvial plain, with maximum havoc being caused by the Kosi river and its feeder channels. The recent floods of the Kosi basin by abrupt migration of the master stream eastwards by 110 kilometers have impacted upon the geography of this densely inhabited region. Migratory trends of the Kosi indicate that neotectonism and local isostatic adjustments are active in the heavily faulted river basin. Annual precipitation receipts also contribute to channel over spillage, and current glacial recession due to global warming in the source regions of the Kosi impact upon the stream s discharge. The channel has shown a marked affinity of following lineaments and faults. It now coincides with the Bhawanipur Fault, while an eastern branch of the river trends towards the Malda Fault. This study seeks to explain changes in the river s morphology in the light of regional tectonic changes and changing climate in the subcontinent. The methodology adopted includes interpretation of satellite imageries, geological reports, flood reports and topographical maps followed by GIS analysis of the spatial shift of the Kosi river. This occurrence supports our earlier hypothesis that the ongoing changes in the surface water configurations, as evinced through remote sensing imageries within a span of twenty years, are due to active neotectonism and current climate change in the entire Kosi Fan Belt area. It is also indicative of heightened seismicity of northern Bihar Plains Keywords: Kosi River, Neotectonism, seismicity, glacial recession, climate change Summary of the paper :- Kosi Floods- Nature induced or Man-made?

2 - Dr. Ashok K. Ghosh, Prof-in-Charge, Dept. of EWM, A.N.College, Patna & Dr. Nupur Bose, Head, Dept. of Geography, A.N.College, Patna The Kosi river system is the only Indian river whose hydrology is deeply influenced both by the regional geological complexities and inputs of annual precipitation and Himalayan glacier melts. This antecedent drainage system is notorious for its migratory trends, resultant flooding, and a huge detrital load of boulders and sand. Entire Kosi fan belt is etched with palaeo-channels that demarcate the east-to-west and back swing of the river from the Epic Age onwards. The recent flooding caused by a breach in the Kosi embankment in Nepal has highlighted the increasing role of geological processes like plate motions, neotectonics, and local isostatic equilibrium in the Himalayan region and the adjacent Ganga basin. Presently, the Kosi has abandoned its westward curving channel for an almost direct north-south flow from the Himalayan base upto its confluence with the Ganga. Modis Image, Aug. 22, 2007 Remote Sensing Image of Kosi Basin- August 2007

3 Modis Image, Sept.02, 2008 Remote Sensing Image of Kosi Basin- September 2008 Remote sensing imageries and geology of the Kosi Fan reveals a number of facts that must be taken into account in future management of the river. These are: The breach of the river embankment occurred on its left bank, which is the cutoff slope of an elbow bend of the river, implying the already existing pressure of the turbulent water descending steeply from the Himalayas. The previous curved shape of the channel in its middle reaches roughly coincided with the underlying Barauni fault, the present flow being parallel and mostly above the eastern Bhawanipur fault located in the basement complex beneath the sediments. It implies that horizontal plate movements of the region is also being accompanied by vertical or isostatic adjustments of the rock layers, thereby making the entire region not only flood-prone but also seismically volatile [ earthquake-prone].

4 Older map of Major Faults in the Kosi Fan Belt: River Kosi flowing through its curved channel along Barauni Fault, now also flowing along Bhawanipur fault towards Ganga. Fears of tectonic changes having caused the breach in the Kosi embankment has been rendered true following report of an earthquake measuring 6.8 on Richter scale that occurred in Tibet on 25 August, Kosi river originates in the Tibetian Himalayas. Any disturbance in the Tibet Plateau has the potential to impact the river s course on account of elastic rebound of the affected rock structures. The stress built up in the last 200 years, may have induced the eastern migration of the channel due to displacement of the crust along the main and transcurrent faults below the river bed.

5 The researchers had presented papers in Australia [2004] and UK [2007] which highlighted the possibilities of the eastward migration of the Kosi river resulting from upliftment of the north-western Bihar and subsidence in the north-east as evinced from remote sensing studies. They had reported first the changing trends of surface water configurations over a period of last two decades, followed by resultant changing land use patterns. It would be extraneous to state that current climate change has been largely responsible for the sudden change in Kosi s direction of flow, in the absence of similar trends in the remaining parallel Himalayan streams in the Indian Plains. Kosi is a highly unstable river, whose detritus is dominated by sand that renders the soil infertile. Historical records of living with the floods in the plains would suggest adjustment of infrastructure and livelihoods to the annual inundation by the Kosi System, by allowing the river to settle into palaeochannels during rainy season and leave the interfluves dry. It requires immediate attention by earth scientists to see the emerging trends of the river Kosi influenced by geological causes, and then decide upon appropriate action.