Two geological factors are ruling in determining the morphological character of the river in Assam.
Firstly, the Himalayan ranges to the north are uplifting at a rate of the order of one meter per Century. Secondly, the whole region is subject to frequent seismic movements and periodic major Earthquakes. The recent earthquakes of 1897 and 1950 in Assam, both of magnitude 8.7 on the Richter scale, were among the largest within historical experience anywhere in the world. The 1950 earthquake seriously affected the territory in Assam causing floods and erosions. A public-sector flood and erosion program to curb the ravages of floods and river bank erosion started in many parts of Assam after declaration of National Flood Protection Policy in 1954. A network of earthen embankment was constructed under the “Food for Work” program from the mid-1950s until the end of the 1070s with setback distances from the river bank. However, the network of earthen embankments did not prove to be a full-proof tool for flood control. In many instances, the embankments prevented the transport of silt to the wide floodplains and riverine wetlands during annual floods when floodwater overflowed banks. As a result, the deposition of silt within the river beds and floodplains within the embankments reduced the effective flood flow area and conveyance capacity of the rivers.
The structural integrity of the earthen embankment system also became vulnerable due to poor cohesive property of the sandy silt material, and led to frequent occurrences of levy breaches and untold miseries to the masses across the region. Many reaches of the embankments failed from erosion. Also, they are partially effective for reasons such as:
Based on the satellite image estimation of area eroded in Brahmaputra for the recent years of 1997 to 2007-2008 (Sarma, 2009), the total land loss per year (exclusive of avulsion) is reported to be 72.5 sq.km/yr. The erosion wiped out more than 2500 villages and 18 towns including sites of cultural heritage and tea gardens affecting lives of nearly 500, 000 people.
Based on records, all suspended sediments (Bed sediments and wash load) is reported as approximately average rate for June to September as 2 million tons/day at Pandu. The average discharge of the Brahmaputra increases from 8500 to 17,000 cubic meters per second as flows are augmented by major tributaries. According to information, high floods occurs on average about every 5 yrs. From 1954 to 2004, but the frequency and the magnitude of floods have been increased in recent years. The average area flooded for the 9 years of highest flood is 2 million ha or 20,000 Square km (say 400 km x 50 km) or 25% area of Assam. The social disruption and cost associated with flooding have been rising.
Embankments built to prevent flood s are only partially effective for numerous reasons:
Photos of Impacts of Flooding and Erosion on Majuli: