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Channel metamorphosis in part of middle Ganga basin,India -A study based on multi-data sets

G. Philip, R. P. Gupta
Department of Earth Sciences University of Roorkee Roorkee-247 667, India


Abstract
The rivers in the middle Ganga basin are known to have an oscillatory character, having changed their courses in the recent times. In the present study, a variety of remote sensing data (Landsat MSS, TM, Metric Camera, Aerial photographs) combined with surface (topographic maps, land use maps etc. and subsurface data (well-log data)) have been used to delineate the fluvial palaeofeatures of the rivers. Digital enhancements of landsat MSS and TM data by ratoing and contrast manipulation have been found to be highly powerful in identification and mapping of the subtale fluvial palaeofeatures. The palaeocourses of the Ganga and the Burhi-Gandak rivers have been reconstructed. Using the disposition and pattern, of various reatures such as oxbow lakes, meander scars and abandoned channels. It is inferred that the Ganga and Burhi Gandak rivers have moved from north to south by 20 and 30 km respectively. Based on the above features, distinct stages of migration of the rivers have been identified and a summary of the fluvial metamorphosis is presented.

Introduction
The rivers are dynamic bodies which oscillate and migrate and thus influence landforms and various local and regional resources such as landuse, groundwater etc. in a region. The Ganga river is one of most important rivers in the Indian sub continent. The middle Ganga basin is especially prone to flooding and river migration. It is observed that migratory rivers invariably leave their footprints behind. The footprints include meander scars, abandoned channels, oxbow lakes and natural levees. These features can be used to re-establish the migration history of the rivers (Baker, 1986 Gregory 1977 & 1983). Aerial photographs and satellite images are particularly useful for mapping palaeofeatures as they are clearly depicted on such synoptic overviews. This gives enough scope for visualizing the complexity of fluvial environment and landscape changes and dynamic geomorphology (Verstappen, 1986). Keeping this in view, an area in the middle Ganga basin, lying around Monghyr, has been selected for the present study. The Ganga is the major river and Burhi-Gandak forms an important tributary in this area (Fig. 1).

Data Sources and Methodology
  1. Data Sources

    The study has been based on a variety of data viz., aerial photographs (black -and -white@ 1:60,000 scale, 1966 coverage). Present address: Wadia Institute of Himalayan Geology, 33-General Mahadeo Singh Road, Dehradun -248 0001, INDIA


    Fig.1: Location map of the study area

    Landsat MSS and TM data products (black-and -white) images, FCCs and CCTs (for 1975, 1982,1983,1984 1985 and 1986) , Metric Camera photographs, topographic maps (year of surveys : 1935 and 1978 with)maps produced on 1:250,000 and 1:50,000) and lithologs of about 25 wells. The observations and interpretations have been controlled by field data.

  2. Methodology

    The remote sensing data have been studied in detail for deciphering the fluvial palaeofeatures and interpreting the plaeocourse. Digitally processed and enhanced Landsat MSS and TM data products, by linear contrast stretching and band ratioing have been found to be highly useful for improved identification of the paleofeatures (Philip et al. 1989). The photographs and image products have been studied at available scales and finally, the observations and results have been geometrically colleated to a common scale.

    The reconstruction of the palaeohistory of the Ganga and Burhi-Gandak rivers has been attempted for a part-stretch of the rivers. It has involved close study of the palaeofeatures such as abandoned channels, meander scars, oxbow lakes, their shape, size orientation etc, the orientation of the palaeofeatures being very important, as it gives valuable clue to the direction of local shifting of the rivers.

    Initially, the river palaeocourse was outlined and reconstructed by suitably interpreting and joining the palaeofeatures (Fig. 2 & 3).. To substantiate the above interpretation, lithologs were studied and fence diagrams were prepared in selected directions. The interpretation drawn from the remote sensing data, field data and litholog data have been analysed and interpreted in a coherent manner.

Fig.2: landform map of the area (prepared from aerial photographs)

Interpretation and Results
A sequential and comparative study of the multidate datatopographic maps, aerial photographs and Landsat MSS and TM imagery was carried out . The changes in observed channel location range from about 0.5-30 km. Based on the observations it was inferred, that the river Ganga experienced severalstages of migration generally towards south, to occupy, its present -day course. The Ganga, in the past must have been flowing approximately 20 km north of its present course. The river has shifted in five major stages southwards punctuated by four episodes of relatively rapid changes in course. The shifting due to rapid changes in course may not leave any renant for long distances in the transverse direction. but within each stage the river is found to have possessed a locally oscillating character, as is indicated by numerous oxbow lakes and palaeochannels in the vicinity of each of the major stages (Fig. 5).

Fig.3: Landsat TM image (Date 25 May, 1986) showing abandoned channels (AC), meander scars (MS) and oxbow lakes (OL) around Biswa and Parihara. These features pertain to the Burhi-Gandak River.


Fig.4 Salient palaeofeatures of the Burhi-Gandak river and their corresponding stages.

Similarly, the Burhi-Gandak river which is a major tributary to the river Ganga in this area also shows evidences of migratory nature in the past, in the form of wide-spread oxbow lakes and palaeochannels (Fig.4) It is estimated that the Burhi-Gandak river has also migrated over a distance of about 30 kms from north to south in the recent past. The palaeofeatures of the Burhi-Gandak river indicated three main stages in shifting of this river (Fig.5).



Stages of shifting of Ganga river Stages of shifting of Burhi Gandak river
i - Stage
ii - Stage
iii - Stage
iv - Stage		 1- Stage
2 - Stage
3 - Stage

Fig. 5: Various stages of shifting of Ganga and Burhi-Gandak rivers

Thus it is found that an integrated study using various data sets such as aerial photographs, images from satellite sensors, surface and subsurface data have provided extremely valuable information in establishing the migratory trend as well as the metamorphosis of the Ganga and Burhi-Gandak rivers in the area.

Acknowledgements
The authors are grateful to Dr. B.B.S. Singhal, Professor, Department of Earth Sciences, University of Roorkee, for providing many fruitful suggestions, GP would like to thank the CSIR, New Delhi for the financial assistance. Bihar Minor Irrigation Department, Patna provided litholog data and assistance in field work. GP would also express his thanks to the Director, WIHG for granting the permission to present this paper.

References
  1. Baker, V.R., 1986 Fluvial landforms. In: Geomorphology from Space, edited by N.M. Short and R.W. Blair Jr NASA SP -486 (Washington D.C.) pp 255-316.

  2. Gregory, KJ., 1977 , The context of river channel changes. In River Channell changes, edited by K.R. Gregory (Chichester:) Wiley Inter Science) pp 1-12.

  3. Gregory, K.J. 1983, Background to Paleohydrology: A perspective (Chichester: Wiley Interscience)

  4. Philip, G., Gupta, R.P. and Bhattacharya, A., 1989, Channel migration studies in Middle Ganga Basin, India, using remote sensing data. International Journal of Remote Sensing, Vol.10 No., pp 1141-1149.

  5. Verstappen, H.Th., 1983, Applied Geomorphology: Geomorphological Surveys for environmental development. Elsevier Science Publishing Co., New York, 432 p.