Origin and some geomorphological changes of Majuli Island of the Brahmaputra River in Assam, India

Abstract

The present study on Majuli, the largest river island of the Brahmaputra, aims at exploring its origin and geomorphological evolution during the last century. According to the earlier official data, in 1901 the island covered an area of 1255 km 2. The databases of the study comprised historical reports, SOI topographic maps of 1917, 1966–1972 and satellite images of 1996 and 2001. The Brahmaputra and one of its major tributaries, the Dihing, once flowed parallel and close to each other. An extreme flood, which occurred around 1750, appears to have diverted part of the flow of the Brahmaputra through the channel of the Dihing about 190 km upstream of its confluence. When the two rivers joined, the intervening land area formed the Majuli Island. According to the first geographical report, Majuli was a cluster of 15 large and numerous small islands in 1792. The description of the island during the 19th century has been given based on various written reports. Whereas the changes in the island during 20th century have been assessed in detail at different parts of the island by superimposing the bank lines of 1917, 1966–1972, 1996 and 2001 and subdividing the island into an array of twenty-six 5′ latitude×5′ longitude blocks. The rate of bank line shift has not been uniform throughout the island in time and space; migration leading to erosion along the Brahmaputra is dominant at the south bank, the maximum rate of retreat being 0.480 km/year. Bank line migration leading to silting is confined chiefly to the north bank due to the Subansiri River, the maximum rate of advance being 0.146 km/year. In 1917, the island had an area of 751.31 km 2, which gradually shrank to 564.01 km 2 in 1966–1972, 453.76 km 2 in 1996 and 421.65 km 2 in 2001. Similar changes were noticed in the length and width of the island. The average annual rate of erosion was 1.77 km 2 from 1917 to 1972, 1.84 km 2 from 1972 to 1996 and 6.42 km 2 from 1996 to 2001, indicating a gradual increase in the rate of erosion in later periods. In 1917, the number of named streams draining Majuli was 49, which decreased to only 7 in 1966–1972. Drainage density was 0.963 km/km 2 in 1917, which decreased to 0.442 km/km 2 in 1966–1972. Similarly, stream frequency also decreased from 0.346/km 2 in 1917 to 0.144/km 2 in 1966–1972. Majuli has a fairly large number of swamps and wetlands. The larger swamps owe their origin to pre-existing rivers. The total number of swamps and wetlands observed during 1917 was 112 covering total area of 20.13 km 2, which decreased to 50 occupying a composite area of 17.88 km 2 during 1966–1972.