A Case Study from Lower Ganga Plains, India

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Geocarto International Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tgei20 Understanding dynamics of large rivers aided by satellite remote sensing: a case study from Lower Ganga plains, India Rajiv Sinha a & Santosh Ghosh a a Engineering Geosciences Group, Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India Accepted author version posted online: 15 Sep 2011. Version of record first published: 26 Oct 2011

To cite this article: Rajiv Sinha & Santosh Ghosh (2012): Understanding dynamics of large rivers aided by satellite remote sensing: a case study from Lower Ganga plains, India, Geocarto International, 27:3, 207-219 To link to this article: http://dx.doi.org/10.1080/10106049.2011.620180

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Understanding dynamics of large rivers aided by satellite remote sensing: a case study from Lower Ganga plains, India Rajiv Sinha* and Santosh Ghosh

Engineering Geosciences Group, Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India (Received 12 June 2011; ﬁnal version received 29 August 2011)

The advent of satellite remote sensing has provided a huge opportunity to geomorphologists to study the temporal dynamics of large rivers. The repetitive coverage of satellite data is an important archive to reconstruct historical-scale dynamics of large rivers and to understand the causal factors. In the lower reaches of the Ganga River around Farakka, natural as well as anthropogenic factors have influenced large-scale dynamics of the river during the last 234 years. The construction of the Farakka barrage was started in 1961 and was completed in 1971 except the feeder canal. The barrage was finally commissioned in 1975 but the serious problems of aggradation both upstream and downstream of the barrage had started much earlier resulting in significant changes in channel morphology and position. The channel upstream of the Farakka barrage has moved towards the east but channel shifting downstream of the Farakka barrage has been erratic. We argue that sedimentological readjustments due to aggradation and bar growth are the major factors influencing the river dynamics in this region. Keywords: river dynamics; aggradation; drainage reconstruction; fluvial geomorphology; floods

1. Introduction A river is a natural waterway, which flows across the landscape from higher to lower elevations, and it is an important component of the water cycle. The rivers are generally fed by precipitation through surface runoff, groundwater and release of stored water in natural reservoirs such as a glacier. But these rivers are also responsible for natural hazards like bank erosion, flooding and lateral shifting.

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 Further, natural as well as anthropogenic factors can alter river channel position and form, flow rate, and bank line characteristics albeit at different spatial and temporal scales. River dynamics is one of the major problems in rivers draining the Ganga plains and is particularly acute in the eastern plains from where some of the most interesting histories of fluvial dynamics have been reported (Gole and Chitale 1966, Wells and Dorr 1987, Sinha 1996, Jain and Sinha 2003, 2004, Sinha 2009, Chakraborty et al. 2010). Various causal factors have been suggested for river dynamics in the eastern Ganga plains ranging from tectonic tilting, sedimentological

*Corresponding author. Email: [email protected]

ISSN 1010-6049 print/ISSN 1752-0762 online Ó 2012 Taylor & Francis http://dx.doi.org/10.1080/10106049.2011.620180 http://www.tandfonline.com 208 R. Sinha and S. Ghosh

readjustments and hydrological variability. Apart from changes in channel positions, large-scale avulsions also cause extensive flooding in areas which are generally not expected to flood. In a recent paper, Sinha (2009) documented a large-scale avulsion of the Kosi in August 2008 resulting in extensive flooding in areas as far as 120 km from the present-day channel belt. Such events make the river management rather challenging in this region. This article is focused on the fluvial dynamics in the relatively less studied Lower Ganga plains in West Bengal, eastern India (Figure 1). Rudra (2010) documented the drainage reconstruction of the Ganga River between Rajmahal and Jalangi for the period 1764–2007 and discussed the science–policy interaction of river migration. The author commented that the river migration in this area is not just a major engineering problem but is a serious social issue impacting land reallocation, population displacement and border dispute. The present article takes this work further and attempts to understand the causal factors of river dynamics in response to natural as well as anthropogenic forcings.

2. Geological and geomorphic setting The Lower Ganga plains are quite distinctive in terms of its geological setting (Figure 2). After draining through a wide alluvial plain in Bihar plains, the Ganga Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012

Figure 1. Location of the study area on the IRS LISS III image of March 2004. The western valley margin of the river is conﬁned by the Rajmahal Hills whereas the eastern side forms a wide valley. Geocarto International 209

Figure 2. Geologic and tectonic setting of the study area (modiﬁed after Narula et al. 2000).

River is suddenly conﬁned on both sides; the right bank is bordered by the outliers of the Rajmahal hills and the left bank is bound by the old alluvium of the Barind Tract. Several tectonic elements have been mapped in this region based on the seismic and gravity surveys carried out by Geological Survey of India (Figure 2) (extracted from the Seismotectonic Atlas of India, Narula et al. 2000) shows that two major faults run nearly parallel to the Ganga River, the Rajmahal Fault along the

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 right bank and Malda–Kishanganj Fault along the left bank; these two faults have forced the river to ﬂow in a relatively narrow valley for *80-km long stretch. East of Malda–Kishanganj Fault, positive gravity anomalies and very shallow basement mark the Malda–Rangapur Ridge. A number of smaller faults are also mapped cutting across the Malda–Kishanganj Fault to the east of the Ganga River. It is clear that a major part of the course of the Ganga River in the study area is geologically- controlled. Between 1594 and 1975, seven earthquakes of moderate size have occurred around Farakka and adjoining regions (Singh and Singh 1989) which suggests that the area is neotectonically active. The Ganga is the main river system of West Bengal and is known to have a long migration history in this region. Channel movements still continue in diﬀerent reaches. As Rennell (1788) wrote ‘Gour, the ancient capital of Bengal, stood on the old 210 R. Sinha and S. Ghosh

bank of Ganges: although its ruins are 4 or 5 miles from the present bank.’ According to Major Hirst (1916), the Ganga shifted due to tectonic activity and mainly by 1515 AD earthquake. Prof. R.K. Mukherjee (1938), the famous historian, mentioned in his book named ‘The Changing Face of Bengal’, that ‘Leaving the hills of Rajmahal, Ganges seemed to have passed northwards through the modern Kalindri and then southwards in the lower course of Mahananda, east of the ruins of ancient Gour’. The Ganga maintained its southeasterly course between Rajmahal and Farakka till the fourth decade of the twentieth century when it started to migrate eastwards (Singh et al. 1980). The Ganga River between Sahibganj and Nawabganj area shows abrupt change in its course. This river was declared as an international boundary in the year 1947 between India and East Pakistan (now Bangladesh). After that the rapid growth of population in this region and large-scale migration from East Pakistan towards the Indian side coupled with unplanned expansion of habitation along the banks of Ganga increased the bank erosional problems many fold. Moreover, erratic alignment of roads and railway network in the region has made the situation worse. Historical accounts (Rennell 1788) suggest that the major stream in this region in the late 17th century was the Tista which, near Jalpaiguri, branched and followed the course of the Depa–Punarbhaba to the west, the Atreye due south and the Little Jamuna to the southeast. All of these rivers discharged into the Ganga. Of these, the Atreye seems to have been the major channel in the late 17th century with the Punarbhaba and Karatoya as other important channels (Rennell 1788). All of these channels now flow into the Brahmaputra. The river courses, particularly of the Tista, changed suddenly in 1787 during a major flood possibly accompanied by renewed uplift and tilting of the Barind surface and slow subsidence of the central Bengal plains (Morgan and McIntire 1959). Presently, the Punarbhaba, Atreye and Little Jamuna mainly carry local runoff water of the uplifted Barind area and some flood flow. The hydrological regime of the Ganga River in the study area is quite distinctive in terms of its fluctuating discharge and sediment load (Rudra 2000a,b). Major engineering projects such as the Farakka Barrage, construction of flood control embankments, bank revetment with boulders, construction of spurs to deflect the impinging current and excessive exploitation of groundwater have affected the natural flow characteristics and dynamics of the river system.

3. Data used and methods

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 In the present study, digital remote sensing data of IRS-LISS III sensor (spatial resolution 23.5 m) obtained from National Remote Sensing Agency (NRSA), Hyderabad were used. The study area is covered in two sheets of Survey of India topographic sheets of 1:250,000 scale. Rennell’s Map of 1776 (Rennell 1788) and US Army survey maps were also used for this study. The speciﬁc details of the remote sensing data, topographic sheets and maps used for the present study are listed in Table 1. The topographic sheet and maps covering the study area were scanned and then imported into the ERDAS format. All 1:50,000 scale topographic sheets were individually ‘georeferenced’ and then a mosaic was prepared. The satellite image was then registered on the topographic sheets with the help of ERDAS IMAGINE PC based software version 8.5, using modiﬁed Everest projection system with the help of Geocarto International 211

Table 1. Data products used for the study.

Data type Details Year/date Source Satellite remote sensing data IRS P6 LISS III Path/row: 107/54 1 March 2004 NRSC Hyderabad Landsat TM Path/row: 139/043 2 April, 2010 NASA website Maps Historical river map Scale 1:250,000 1776 Rennell (1788) River map Scale 1:250,000 1916 West Bengal Gazetteer Topographic sheets Scale 1:250,000 1955 US Army Survey Maps District planning map Scale 1:250,000 1971 NATMO, Kolkata Topographic sheets Scale 1:250,000 1971 Survey of India and 1:50,000

90 ground control points (GCPs). The root mean square error was 0.932 which is within the acceptable limits. Mapping of the Ganga River channel was carried out using the corrected image, topographic sheets and published historical maps coupled with field verification. SRTM based digital elevation models were also used to generate topographic profiles across the river and the adjoining floodplain. All data was integrated in a GIS environment to understand the controls and mechanism of river dynamics. For a detailed analysis of the dynamics of Ganga River, and to understand the causal factors, the study area has been divided into two windows (Figure 1). Window 1 includes the river reach between Rajmahal and Farakka and Window 2 includes the river reach between Farakka and Khandua. The selection of these windows is guided by the location of the Farakka barrage which is a very large engineering structure on the Ganga River and is likely to have influenced the river dynamics in a major way.

4. Channel morphology and topographic analysis Systematic description of migration events is presented in the following sections. Apart from studying historical scale changes in river conﬁgurations, variation in morphological parameters such as sinuosity and braid-channel ratio (after Friend and Sinha 1993) were also measured for these two windows from the relevant geomorphic maps prepared using satellite images and other historical data (Figure 3). The contrast in planform parameters in two windows is remarkable. The reaches in

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 window 1 show relatively low values of sinuosity (1.09–1.31) and moderate variation in braid-channel ratio (1.23–3.15) during the period of study. The most remarkable change in braid-channel ratio for this window occurred in the period 2004–2010. In contrast, the window 2 has similar sinuosity (1.05–1.28) but a remarkably high braid- channel ratio (2.09–3.21) throughout the period of study. The implication of such morphological diversity and variability will be discussed later. The SRTM DEM for the Lower Ganga valley (Figure 4(a)) clearly shows that the Ganga River is conﬁned by the Barind tract in the east with a maximum elevation of *40 m and by the Rajmahal Hills in the west with an average elevation of *290 m. The Rajmahal hills mark the southern edge of the Ganga basin in the region and the Ganga River ﬂows closest to this edge in this region in its entire journey from the Himalaya to the Bay of Bengal. Downstream of Farakka, the river swings 212 R. Sinha and S. Ghosh

Figure 3. Temporal variability of sinuosity and braid-channel ratio of (a) Window 1 and (b) Window 2 in the study area.

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 Figure 4. (a) Digital Elevation Model (DEM) derived from the SRTM data; (b) Elevation proﬁle along A–B transect; (c) Elevation proﬁle along C–D transect.

northwards and resumes the axial course before finally meeting the Brahmaputra River. Two representative sections (AB and CD) across the Ganga valley in this region (Figure 4(b) and (c)) reveal the valley configuration. The southwestern edge defined by the Rajmahal hills is conspicuous in both profiles and the north eastern valley margin is extremely flat and merges with the floodplain of Ganga River. The width of this asymmetric valley is variable, ranging from 35 to 50 km, but the active channel belt is close to 10 km wide in most reaches. The maximum depth of the Geocarto International 213

Ganga River channel appears to be close to 10 m but is significantly different along the different transects.

5. Systematic reconstruction of fluvial dynamics in the Lower Ganga plains 5.1 Window-1: upstream of Farakka barrage Systematic reconstruction of channel positions for the period 1776–2004 (Figure 5 (a)–(d)) indicates gradual shifting of the Ganga River towards the eastern side. Between 1776 and 1955, the Ganga River moved eastward and few side bars developed (Figure 3(b)). Moreover, the historical sinuous channel around Farakka was transformed into a straight channel which is manifested as an overall reduction of sinuosity from 1.12 to 1.09 but a more significant reduction in braid-channel ration from 1.80 to 1.23 (Figure 3(a)). Between 1955 and 1971, the channel around Farakka region doesn’t show any significant change but in the reaches upstream of Radhanagar village, the channel moved eastward by *10 km and formed a large meander loop. Further, a comparison between 1971 and 2004 maps shows more remarkable changes in the study area (Figure 5(d)). Immediately upstream of Farakka, the narrow, straight channel of 1971 was transformed into a very broad channel by 2004 encompassing numerous mid channel bars. Significant changes are also noticed downstream of Rajmahal where meandering became more pronounced by 2004. As a result, significant deposition occurred on the concave side of the channel producing a large point bar complex. Later, numerous secondary channels dissected this point bar complex resulting in a highly braided reach. This is also evidenced by a significant increase in braid-channel ratio in this window between 1971 and 2004 Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012

Figure 5. Systematic reconstruction of channel conﬁguration for Window 1 for the period 1776 to 2010 (RJ – Rajmahal, P – Phudkipur, R – Radhanagar, F – Farakka, M – Manikchak, PN – Panchanandpur, J – Jagannathpur, G – Gansaipur, A – Alinagar, B – Baishnabnagar, GK – Gopal Kagmari, MB – Mothabari, MD – Mahadipur). 214 R. Sinha and S. Ghosh

(Figure 3(a)). The 2010 map of this window shows that the main channel of the Ganga has been fairly stable between 2004 and 2010. However, increased aggradation has resulted in transformation of mid channel bars into a large island. In addition, a large side bar has developed immediately downstream of Rajmahal. Another interesting aspect of this window is the dynamics of the Pagla river which is a distributary or a spill channel of the Ganga River and was ﬂowing almost parallel to the Ganga River until 1955 (Figure 5(b)). The Ganga River started to move towards the east around 1955 and partially captured the downstream reaches of the Pagla river by 1971 (Figure 4(c)). By the year 2004, the Ganga River captured a large part of the Pagla river except for a small stretch which is in moribund state at present (Figure 4(d)). Figure 5(a)–(e) also reveals that the Pagla itself has been a stable system but its course has been strongly inﬂuenced by the dynamics of the Ganga River. The Ganga River at Manikchak has been undergoing extensive erosion along the left bank even though it is strongly protected. Field surveys around Manikchak reveal extensive erosion through bank failure all along the left bank (Figure 6(a)). An exposed section at Manikchak reveals two sand bodies; the lower one, *2 m thick (base not exposed), is overlain by a channel margin/levee sequence consisting of climbing cross ripple laminations and then overbank mud. The upper, thin sand sheet apparently belongs to the modern Ganga River (Figure 6(b)). It implies that either a major or comparatively smaller secondary channel existed at this location prior to the eastward journey of the river.

Figure 6. (a) Bank failure along the left bank of the Ganga at Manikchak, (b) bank stratigraphy at the same location provides clues about recent ﬂuvial dynamics. A thick lower sand body below the modern channel margin/overbank sequence suggests the former position

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 of the Ganga NE of its present course. Black arrow shows the present-day ﬂow direction.

5.2 Window-2: downstream of Farakka barrage Systematic reconstruction of channel positions in this window indicates gradual shift of the Ganga River towards the west between 1776 and 1916 (Figure 7(a) and (b)) and then towards the east until 2004 (Figure 7(c)–(e)). In the year 1776, the Ganga River channel was narrow and meandering in nature. Afterwards, the river shifted westward by more than 10 km and became broader by the year 1916 (Figure 7(b)). The channel multiplicity also increased as reﬂected in a marked increase in braid- channel ratio (Figure (3)). This was a very major shift and several villages such as Bakhrabad and Jagannathpur which were located far away from the right bank of Geocarto International 215

Figure 7. Systematic reconstruction of channel conﬁguration for window 2 for the period 1776 to 2010 (B – Bakhrabad, C – Char Basudevpur, D – Dhulian, F – Farakka, J – Jangipur, JG – Jagannathpur, K – Khandua, N – Nimtita, P – Panka, R – Radhakantapur).

the river in 1776 were now located on the left bank of the river. A comparison of maps between 1916 and 1955 indicates that channels did not shift any further during this period. Between 1955 and 1971, the channel form was modified significantly clearly due to the interventions related to the construction of the Farakka barrage. The 1971 map shows the sinuous Ganga channel of 1955 with multiple bars changed to a planform with two distinct channels separated by a large alluvial island resulting in an overall decrease in braid-channel ratio (Figure 3). Moreover, a large sand bar developed in the lower reaches resulting in a westward convexity of the channel. Several villages such as Bakhrabad, Jagannathpur and Radhakantapur became a part of the large island. The 2004 map reveals significant engineering interventions downstream of the Farakka barrage resulting in a narrow and nearly straight channel. Downstream of Nimita village, a large eastward convexity and growth of a

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 large point bar are noticeable on the 2004 map (Figure 7(e)). Some of the villages such as Bakhrabad and Jagannathpur were now located outside the channel belt but new villages such as Panka were engulfed by the river. One of the worst-aﬀected locations due to river migration and bank erosion is Dhulian town (9 km downstream of the Farakka barrage). The Ganga has a highly convex bend at this location and the right bank is strongly embanked with large boulders to prevent bank erosion. The entire Dhulian town and population had to be shifted by more than 2 km in the last 25 years due to the migration of the Ganga River southwards. Bank failure is commonly observed around Dhulian and the boulder protection has been destroyed at several places (Figure 8(a)). Large rotational failures were observed at several points along the bank during the ﬁeld survey (Figure 8(b)). At some locations cracks parallel to the bank line 216 R. Sinha and S. Ghosh

Figure 8. Severe bank erosion around Dhulian. (a) Boulder protection for banks destroyed, (b) Rotational bank failure in the nearby area. Black arrow shows the present-day ﬂow direction.

were observed 2–3 m inland of the bank line indicating future sites of rotational failure.

6. Discussion Our study shows that the Ganga River has been remarkably dynamic in the study area except for the reaches close to the Rajmahal Hills. In window 1, the general trend of migration is towards the east. Since the Rajmahal Hills form a natural barrier on the western side, the river has no choice but to move eastward as a consequence of natural adjustment in response to hydrological processes. However, channel form in this window has undergone significant modifications particularly after 1961 (as seen on the maps of 1971 and later) and this is certainly a manifestation of the engineering interventions related to the construction of the Farakka barrage. Significant amounts of sediments started to get trapped upstream of the barrage perhaps soon after the construction started in 1961 and large sand bars formed just upstream of Farakka barrage particularly along the western margin. In consequence to bar growth, the river started to migrate further towards the east and developed a conspicuous bend towards NE which continues till date. This bend raised the concern for the river flanking the barrage and has led to extensive bank protection measures in recent years both upstream as well as downstream of the barrage. The dynamics of the Ganga River in window 2 has primarily been caused by two

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 mechanisms: (a) meander migration of sinuous channels and (b) sedimentological readjustment. A dominant westward movement between 1776 and 1955 was primarily caused by meander cut-offs. A major meander loop in the upper reaches was cut-off by 1916 but a new set of meander loops developed by 1955. The SRTM based DEM of this window reveals a very flat topography which would mean that geomorphic threshold crossing for meander development and their abandonment could be quite low and a small change in the hydraulic regime may result in rapid morphological changes. A decrease in sinuosity and a concomitant increase in braiding index during the period 1776–1955 reflect this natural behaviour of the river. In the post-1955 period, the fluvial dynamics was strongly influenced by sedimentological readjustments primarily triggered by the engineering interventions related to the Farakka barrage and other control structures such as embankments on Geocarto International 217

both sides. A large island and two separate channels on 1971 map (Figure 7) are clear manifestations of these interventions. Similarly, a prominent meander developed in 2004 in the downstream reaches clearly in consequence to the construction of embankments to channelize the river and additionally to safeguard the river flanking the barrage. The large island disappeared by 2004 but aggradation continued in downstream reaches. The large meander which developed around 2004 continues to migrate towards NE. It is likely that a cut-off would occur soon and several of the secondary channels are active already. Similar controls and mechanisms of fluvial dynamics have been reported in the adjoining plains of north Bihar where the Kosi river has shifted over its megafan surface from west to east over a distance of about 150 km in the period of 200 years (Gole and Chitale 1966, Wells and Dorr 1987, Chakraborty et al. 2010) and a major avulsion occurred in 2008 (Sinha 2009, Chakraborty et al. 2010). Further west, the Gandak River has also shifted westward by about 80 km in the last 5000 years and this unidirectional shifting was related to the slow eastward neotectonic tilting of the megafan (Mohindra et al. 1992). Several smaller rivers draining the Gandak–Kosi interfluve region in north Bihar plains have also been shown to be migratory and channel movements through avulsions and cut-offs have been recognized in most of them (Sinha 1996, Jain and Sinha 2003, 2004). A closer comparison is from the reaches around Monghyr, *200 km upstream of the study area, where the Ganga has been shifting towards south mainly through meander migration inferred from a series of abandoned meanders and spectacular ox-bow lakes along the northern bank of the river (Phillip and Gupta 1993). Given the tectonic setting of the study area, it is important to discuss the role of neotectonics on fluvial dynamics in this region. Figure 2 shows that the area is bounded by faults running parallel and across the channel belt. However, most of these are sub-surface faults and there is very little evidence of their recent activity. On the western side, the Rajmahal Hills are known to be stable. There are also no geomorphic signatures of active tectonics and a few recent earthquakes in the adjoining areas have not affected the river in the study area. The gravity anomaly contours reveal that the channel belt is flowing through a narrow valley bounded by faults (Figure 2). Apart from this, there seems to be no other geologic control on the river system in this region and we do not think that active tectonics have played any major role in influencing the fluvial dynamics at the temporal scale presented in this study. Tectonic controls have however been pointed out in the adjoining north Bihar plains by earlier workers (Jain and Sinha 2003, 2004) who documented decadal-scale avulsions in the Baghmati river over the last 250 years, and used the

Downloaded by [Indian Institute of Technology Kanpur] at 19:31 02 July 2012 term ‘hyperavulsive’ to describe such rivers. These areas are however much closer to the Himalayan front and several recent earthquakes (1934, 1886) have been documented in the region.

7. Concluding remarks Channel change is an important aspect of geomorphological evolution and habitat dynamics in large alluvial rivers. The lower reaches of the Ganga in West Bengal show signiﬁcant dynamics in terms of channel position as well as form in the last 234 years even though the river ﬂows through a rather narrow valley bounded by Rajmahal Hills and Barind Tract to its west and east, respectively. Although the Ganga River has been naturally migratory in this region, the engineering 218 R. Sinha and S. Ghosh

interventions namely, the Farakka barrage and associated structures have made the situation worse. The river has been migrating to the east in the reaches upstream of the Farakka barrage and to the west in the reaches downstream of Farakka. The apprehensions of the river flanking the barrage have forced more and more interventions in recent years. Unfortunately, these measures have only shifted the trouble to downstream reaches and have significantly increased the aggradation within the channel belt. The reaches of the Ganga downstream of the barrage also form the international boundary between India and Bangladesh and such large-scale dynamics adds to the land disputes between the two countries. The situation remains grim till date and long-term solutions incorporating geomorphic understanding of the river have been lacking in river management strategy. It is important to realize that river dynamics is a natural behaviour of the river and it is crucial to accurately map the extent of migration and reaches prone to migration. This extent must be defined as the ‘space’ for the river and the concept of floodplain zoning must be seriously pursued. This is not only crucial for saving a large population from the misery of river dynamics and floods but is also important for improving the river health.

Acknowledgements The authors are thankful to the Central Water Commission for providing access to the Farakka barrage during ﬁeld work. One of the authors (SG) received fellowship from Indian Institute of Technology Kanpur (IITK) during his Master’s work and he thanks the Institute for the same. The authors are thankful to the editors of this volume for inviting them to write this article. The senior author (RS) was a visiting professor at Indian Institute of Science (IISc), Bangalore while this article was written and the institutional support is thankfully acknowledged. Ms. Lipi Basu, Mrs. Shikha Sinha and Mr. Haridas Mohanta are thanked for their help in the preparation of some of the ﬁgures.

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