Anthropocene metamorphosis of the Indus Delta and lower floodplain

Abstract

Abstract The Indus River/Delta system is highly dynamic, reflecting the impacts of monsoonal-driven floods and cyclone-induced storm surges, earthquakes ranging up to Mw = 7.8, and inundations from tsunamis. 19th century Indus discharge was likely larger than today, but upstream seasonal spillways limited the maximum flood discharge. Upstream avulsions during the 2010 flood similarly reduced the downstream discharge, so that only 43% of the floodwaters reached the river mouth. The present-day Indus River is wider with larger meander wavelengths (∼13 km) compared to the 4–8 km meander wavelengths for the super-elevated historical channel deposits. The Indus River is presently affected by: (1) artificial flood levees, (2) barrages and their irrigation canals, (3) sediment impoundment behind upstream reservoirs, and (4) inter-basin diversion. This silt-dominated river formerly transported 270+ Mt/y of sediment to its delta; the now-transformed river carries little water or sediment (currently ∼13 Mt/y) to its delta, and the river often runs dry. Modern-day reduction in fluvial fluxes is expressed as fewer distributary channels, from 17 channels in 1861 to just 1 significant channel in 2000. Abandoned delta channels are being tidally reworked. Since 1944, the delta has lost 12.7 km2/y of land altering a stunning 25% of the delta; 21% of the 1944 delta area was eroded, and 7% of new delta area formed. The erosion rate averaged ∼69 Mt/y, deposition averaged ∼22 Mt/y, providing a net loss of ∼47 Mt/y particularly in the Rann of Kachchh area that is undergoing tectonic subsidence.