HYDROLOGICAL VARIABILITY AND LANDSCAPE EVOLUTION IN ALLUVIAL RIVER SYSTEM: AN EXAMPLE FROM THE GANGA PLAINS, INDIA

Hydrological controls on landscape evolution of a river basin provide an understanding of river response to climate change. In the present contribution, landscape variability in one of the largest alluvial plains in the world, the Ganga plains, is analysed from hydrological data related to channel transport capacity and sediment load.

The Ganga river originates from the Higher Himalaya and drains the alluvial plains of northern and eastern India for a total length of 1900 km. Numerous tributaries join the Ganga on its journey to the Bay of Bengal, generating a system of coalescing alluvial plains. The rivers draining the western part of the Ganga plains are incised and stable, and are characterized by infrequent overbank flooding. In contrast, the rivers draining the eastern part are aggrading and exhibit frequent channel migration and extensive overbank flooding. In the eastern part, the Kosi river has shifted 110 km westward in the last 200 years across its alluvial megafan. Studies on an eastern interfan river system, the Baghmati, have shown 8 major avulsions in the last 230 years, indicating a decadal scale of avulsion. These avulsion processes are related to high sediment load in the river channel and local neotectonic movements.

These observations suggest geomorphic diversity within the Ganga plains, and this is correlated with sediment supply-transport capacity relationship, which in turn is governed by upstream controls. Rivers in the western plains are characterized by higher specific stream power (40-43 W/m²) and lower sediment load (60-637 tkm^-2yr^-1), whereas rivers in the eastern part are characterized by lower specific stream power (6.36-20 W/m²) and higher sediment load (647-2774 tkm^-2yr^-1). Higher specific stream power with respect to sediment load in western rivers is responsible for channel incision, whereas higher sediment supply against less specific stream power in eastern rivers is responsible for aggradation process. Further, high sediment supply in eastern rivers is related with higher rainfall and uplift rate in the hinterland area, and high susceptibility of erosional processes to rainfall. In summary, upstream processes in the basin area are responsible for geomorphic diversity within the basin.