Abrasion-set limits on Himalayan gravel flux

• Published in: Nature (London) 2017-04, Vol.544 (7651), p.471-474
• Main Authors: Dingle, Elizabeth H., Attal, Mikaël, Sinclair, Hugh D.
• Format: Article
• Language: English
• Subjects: 704/2151/215; 704/2151/3930; 704/4111; Abrasion; Aggradation; Catchment areas; Earthquakes; Environmental risk; Exports; Fluctuations; Grain size; Gravel; Humanities and Social Sciences; letter; Lithology; Mountains; multidisciplinary; Population density; Provenance; Rivers; Sand; Science; Sediments; Seismic activity; Soil erosion; Upstream
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature22039

The amount of coarse gravel transported out of the Himalayan mountains by rivers is insensitive to catchment size, because the majority of gravel sourced more than 100 kilometres upstream of the mountain front is abraded into sand before it reaches the Ganga Plain. Finely ground Himalayan gravel Understanding what happens to coarse gravel during riverine transport is important for forecasting the response of rivers to large sediment supplies triggered by earthquakes or storms. This paper finds that the supply of gravel feeding the Ganga Plain from rivers draining the central Himalayan mountains is approximately constant. The observation can be explained by the wearing down of stones during fluvial transport so that most of the gravel that originates more than 100 kilometres upstream is converted into sand by the time it reaches the plain. The authors suggest that earthquake-induced sediment pulses are unlikely to drive increased gravel deposition at the mountain front. Instead, the sediment surge would result in an influx of sand, leading to distinct deposition patterns and affecting flood risk in the low-relief Ganga Plain. Rivers sourced in the Himalayan mountain range carry some of the largest sediment loads on the planet 1 , yet coarse gravel in these rivers vanishes within approximately 10–40 kilometres on entering the Ganga Plain (the part of the North Indian River Plain containing the Ganges River). Understanding the fate of gravel is important for forecasting the response of rivers to large influxes of sediment triggered by earthquakes or storms. Rapid increase in gravel flux and subsequent channel bed aggradation (that is, sediment deposition by a river) following the 1999 Chi-Chi and 2008 Wenchuan earthquakes 2 , 3 , 4 , 5 , 6 , 7 reduced channel capacity and increased flood inundation 3 . Here we present an analysis of fan geometry, sediment grain size and lithology in the Ganga Basin. We find that the gravel fluxes from rivers draining the central Himalayan mountains, with upstream catchment areas ranging from about 350 to 50,000 square kilometres, are comparable. Our results show that abrasion of gravel during fluvial transport can explain this observation; most of the gravel sourced more than 100 kilometres upstream is converted into sand by the time it reaches the Ganga Plain. These findings indicate that earthquake-induced sediment pulses sourced from the Greater Himalayas, such as that following the 2015 Gorkha earthquake 8 , are unlikely