Megafans of the Gangetic Plains, India

Abstract

The Eastern Gangetic Plains (EGP) hosts three actively aggradational megafans, the Teesta, Kosi, and Gandak, which are directly fed by rivers draining the Himalayan Mountains in the north; and a fourth, the Sone, which is fed from the Indian shield in the south. Topography in the Western Gangetic Plains (WGP) consists instead of a 900-km-long and 100-km-wide raised interfluve between the incised valleys of the Yamuna and Ganga rivers. None of the commonly-advocated megafan criteria, such as mappable fluvial sediment entities with an apex, distributary drainage, convex-up transverse topographic profiles, or a distinct fan boundary with a break in slope, seem to apply. There are thus no active megafans in the WGP. While not ruling out the possible occurrence of relict megafans, evidence suggests that the WGP landscape is essentially a coalescing floodplain in a valley–interfluve setting. Contrasts between the EGP and WGP are controlled by (i) differential late Quaternary and Holocene basin subsidence, which governs regional-scale variations in accommodation space; (ii) along-strike tectonic and climatic variability, primarily reflected in differential uplift rates, rainfall gradients, mountain-front tectonics, and river exit-point spacing; and (iii) hydrological characteristics of the feeder channel manifested as variability in stream power and sediment flux.