Stability of output fluxes of large rivers in South and East Asia during the last 2 million years: implications on floodplain processes

We compare the present‐day sediment discharge (solid phase) of some of the largest rivers in Asia to the average discharge deduced from the mass accumulated in several sedimentary basins during the Quaternary. There is a very good correlation, especially for the largest rivers: the Ganges–Brahmaputra, the Changjiang, the Huanghe and, to a lesser extent, the Indus and the Zhujiang. This suggests that present‐day average discharge at the outlet has remained constant throughout the Quaternary at least for very large rivers (drainage area of the order of 10⁵–10⁶ km²). This, in turn, suggests either that continental denudation of large Asian catchments has remained on average constant, implying a strong tectonic control on erosion during the Quaternary, or that the river network has the ability to buffer changes in hillslope erosion or in sea‐level in order to conserve the total discharge at the outlet. We show how this buffering capacity relies on the characteristic reaction time‐scale of Asian alluvial plains (of the order of 10^5–6 years), that is, much higher than the time‐scales of the Quaternary climate oscillations (of the order of 10⁴ years). A short‐term perturbation originating in hillslopes will be diluted by the floodplain. At the outlet the signal should have a longer time span and a smaller amplitude. In the same manner, an alluvial plain should not instantaneously react to a 10⁴‐year sea‐level drop because of its inertia. Along with long‐term tectonic control we infer this buffering to be the main cause for the average constancy of sediment yield of large Asian rivers during the Quaternary.