Morphotectonic evolution of the Majuli Island in the Brahmaputra valley of Assam, India inferred from geomorphic and geophysical analysis

• Majuli, the largest colonised riverine island has eroded by 3.1 km2/yr (1915–2005).
• Geophysical datasets show a strong role of tectonics behind the evolution of Majuli.
• Three stage evolution — pre-bypass upliftment, Majuli formation and abandonment.
• Presently Majuli in the abandonment stage and Dibru Saikhoa in the middle stage.

The Majuli Island, located in the upper reach of the Brahmaputra valley in Assam (India), has reduced in its areal extent from 787.9 km2 to 508.2 km2 during the period 1915–2005 (35.5% reduction). This amounts to severe average erosion of 3.1 km2/yr. All efforts so far to save the island have failed to achieve the desired redress. The engineering approach of ‘Save Majuli’ action plans has focused on quarantining the island from the influence of the Brahmaputra River rather than designing long-term process-based solutions anchored on proper understanding of evolution of the relic island. The existing geomorphic model for the evolution of the Majuli Island related its genesis to the great earthquake (M 8.7) in 1750 during which a much smaller palaeo-Brahmaputra developed an anabranch and captured the Burhi Dihing River. The intermediate land-locked area thereby became the Majuli Island that is constituted primarily of the older floodplain deposits. We demonstrate that the evolution of the Majuli Island has been influenced by fluvial morpho-dynamics, as well as basement configuration and tectonic controls. Thus, the landform called the Majuli Island cannot be explained as a simple fluvial geomorphic feature. Rather, it represents an outcome of tectono-geomorphic process having strong subsurface control. We have investigated the influence of geomorphic parameters including channel belt area (CHB), channel belt width (W), braid bar area (BB), channel area (CH), thalweg changes and bankline migration on the trend of erosion of the Majuli Island. Integration of geophysical evidence from seismic data and the surface morphological changes suggest that the Majuli Island and other similar landforms represent structural ‘highs’. Morpho-tectonic evolution of these islands has involved three stages- pre-bypass uplift, Majuli formation and abandonment. The Majuli Island in the Brahmaputra valley is presently passing through the abandonment stage and is gradually being incorporated within the flood plain of the valley.