In India, state-wise the drainage area of the Brahmaputra River and its tributaries is shown in Table 4. The Assam and West Bengal portion of the Brahmaputra River basin are mostly plain areas, whereas Arunachal Pradesh, Meghalaya and Nagaland parts are mainly covered by mountain ranges, hills and narrow valleys. In India, among all Eastern and North-eastern states, Assam faces the most severe brunt of Brahmaputra bank erosion.
|States in India||Drainage area (km2)||Percentage of state area in|
|Brahmaputra River basin|
|Arunachal Pradesh||83 740||100.00|
|West Bengal||12 585||14.18|
According to the records of the last century, the Assam valley portion of Brahmaputra River occupied around 4000 km2 in the 1920s, which is now around 6000 km2 (Phukan et al., 2012). The satellite image estimation shows that the total land loss per year due to erosion of Brahmaputra ranges from 72.5 to 80 km2/year during 1997 to 2007 – 2008 (Figure 5*). Bank erosion has wiped out more than 2500 villages and 18 towns including sites of cultural heritage and tea gardens, affecting the lives of nearly 500 000 people.4*). Even though the Ganges is a long river carrying huge discharges of up to 70 000 m3/s, the erosion problems are confined to relatively few places. The flood and erosion problems are serious in the lower Ganges region, particularly in West Bengal.
The River Ganges enters West Bengal near Rajmahal in Jharkhand state and then it flows in a South-easterly direction within West Bengal (Figure 6*). After flowing through Malda district it divides into two branches near North of Dhulian in the Murshidabad district. One branch enters Bangladesh as the Padma, while the other flows through West Bengal as the Bhagirathi River and Hooghly River in Southern direction. The Bhagirathi is the main river in West Bengal, which flows past the Murshidabad, Nadia, Burdwan and Hooghly districts and contributes to a dominant irreparable loss of land each year.
Bank erosion is a common phenomenon in any uncontrolled channel of the deltaic tract. But the case of the River Ganges in West Bengal is quite different. The upstream is so clogged with sediment that the river is compelled to alter its course. The course of the Ganges along the Northern river-front of West Bengal has been fast changing due to unabated bank erosion, especially over the last few decades. The Ganges in Malda district in the upstream of Farakka Barrage has been undergoing extensive erosion along the left bank even though it has been strongly protected (Figure 7*).
However, the erosion problem is not only restricted in the upstream section, it is also widespread throughout its course in West Bengal. One of the major reasons is Farakka Barrage. Farakka Barrage is a dam on the Bhagirathi River located in West Bengal state in India. It is located about 10 km from the border to Bangladesh. The dam was built to divert the Ganges River water into the Hooghly River during the dry season in order to flush out the accumulating silt, which was a problem at the Kolkata Port on the Hooghly River. Now it is well accepted that the intensity of the Ganges-bank erosion in West Bengal has increased after the construction of Farakka Barrage (Rudra, 2010). Official reports show that on an average 8 km2 of land are engulfed annually by the river in West Bengal.