Documenting spatial diversity and complexity in a large tropical river system: implications for river health and management

The Ganga River system in India is a large, complex system consisting of several long tributaries, some >1000 km, from two distinct hinterlands – the Himalayas to the north and the cratons to the south. Traversing through a diverse climatic regime across the Plain and through precipitation zones ranging from 600 mm/year near Delhi to 1200 mm/year in the eastern plains, the Ganga River system has formed very diverse landform assemblages in its major domains that include a) hinterlands, b) northern Plain, c) southern Plain, d) main valley system, e) interfluve systems, and f) major tributaries. Using satellite images, mapping of the Ganga River channel belt and its active floodplain from Gangotri (source) to Farakka (upstream of its confluence with the Brahmaputra) has brought out extreme diversity in channel form, active floodplain and valley margin characteristics, and reach-scale fluvial processes.

Following a modified River Style Framework, we have recognized a total of 10 different river styles for the trunk river from Gangotri to Farakka based on a) landscape setting, b) channel and active floodplain properties, and c) channel planform parameters. The mountainous stretch is characterized by steep valleys and bedrock channels, and is dominated by large-scale sediment production and transport through hill slope processes. The alluvial part of the river is characterized by 8 different styles of varying reach lengths (60-300 km) many of which show sharp transitions in landscape setting e.g. from piedmont to valley-interfluve and from alluvial to craton margin. Each river style in the alluvial reaches has a distinctive set of morphological parameters primarily driven by the dominant fluvial process operating in that stretch such as channel instability and river dynamics, channel incision or aggradation and frequent flooding. Our study has provided a physical template for characterizing the complexity of a large river system and for reach-scale geomorphic assessment of river health, which is the first step towards developing ecology-based river management strategy.