12-14 September, 2011, Lucknow - Earth Science India

National Conference on Science of Climate Change and Earth’s Sustainability: Issues and Challenges ‘A Scientist-People Partnership’
12-14 September, 2011, Lucknow
study area forms a segment of Mid-Ganga Plain around confluence of Ganga, Yamuna
and vedic Saraswati rivers at Allahabad. Present study revolves around two interrelated
aspects of fluvial geomorphology in response to climate change: (I) geomorphic
surfaces, and (II) neotectonics and pattern of channel shifting.
Geomorphic surfaces
The wide range of geomorphic features present in the Ganga Plain, their different
relief and spatial distribution strongly indicate their genesis in climatic conditions
different from what prevails today. Hence, the water budget and base level conditions
were also in all probability in variance with what exist at present. In the study area,
three distinct geomorphic surfaces i.e. T 0 , T 1 and T 2 have been recognised with the help
of satellite imagery. The regional plateau or upland is a terrace (T 2 ) form of higher
planer surfaces, and might have formed during a major period of humid climate leading
to widespread alluviation and planation. T 2 – Surface is characterised by tightly
meander of high amplitude channel remnants indicating a higher water budget during
active phase in the past. It is now well established that there have been several sea level
fluctuations ever since Late Quaternary. The last sea level low stand was around 18 Ka
BP and a rapid sea level rise reached a maximum around 6 Ka BP and since then the
rise in sea level is minimal. The “Terrace” surfaces observed in the Gangetic Plain are
considered to have formed due to the sea level changes. During the last sea level low
stand of 18 Ka BP, extensive fluvial drainage developed within the alluvium, which
drained the high plateau-surface. The rise of the sea level after that resulted in the
lowering of the hydraulic gradient, and consequent reduction of drainage area. Most of
the drainages were abandoned or filled up and the imprints of such are now observed on
the plateau surface throughout the Ganga Plain. The Ganga river was entrenched deeper
into its T 1 - Surface during the last sea-level low stand and adjusted to palaeo base level
of erosion till the beginning of last major sea-level rise. T 0 - Surface of the Ganga River
has only developed during the last phase of sea-level rise and has been continued till
date with minor fluctuations at places.
Neotectonics and pattern of channel shifting
Neotectonic activities of the recent past and even the present time are mostly
responsible for present day landscape geometry. The parameters used in identification
of neotectonic activity in the Ganga Plain are: (i) preferred alignments of rivers; (ii)
sudden change in the direction of the river course; (iii) nick points; (iv) straightening of
the river course; and (v) presence of escarpments and asymmetrical terraces. A few
indirect evidences of neotectonic movements are observed in the area possibly in
relation to the isostatic adjustment. The deformation structures in clay beds, tilting of
beds in the Banda Older Alluvium (BOA) in Yamuna river section near Deoria and
sagging of horizontally bedded channel bar sections in the Ganga river near Mawaiya,
straightening and abrupt change in the course (as in Ganga river before confluence),
widening of low level terrace zone (T 1 ) of the Ganga between Dubwal and Diha (after
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