Abrasion-set limits on Himalayan gravel flux

Abstract

Rivers sourced in the Himalayan mountain range carry some of the largest sediment loads on the planet1, 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 earthquakes2,3,4,5,6,7 reduced channel capacity and increased flood inundation3. 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 earthquake8, are unlikely to drive increased gravel aggradation at the mountain front. Instead, we suggest that the sediment influx should result in an elevated sand flux, leading to distinct patterns of aggradation and flood risk in the densely populated, low-relief Ganga Plain.

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Figure 1: Study area and simplified geological map of the Ganga basin.
Figure 2: Gravel flux estimates.
Figure 3: Catchment and pebble lithology.
Figure 4: Abrasion scenarios for the Kosi (top panels; trans-Himalayan) and Bakeya (bottom panels; foothill-fed) rivers.

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Acknowledgements

We thank V. Singh, A. Gajurel, J. Stewart, F. Bowyer, K. Maheswari, D. Basuroy, A. Sarkar, B. Sitaula and Apex Adventure, and the Nepalese Department of Mines and Geology for their cooperation and logistical support in the field. C. Paola and E. Garzanti provided comments that helped to improve the manuscript. We are also grateful to the International Association of Sedimentologists, the British Society for Geomorphology and the Edinburgh University Club of Toronto for their financial support of the fieldwork. This study formed part of a Natural Environment Research Council (NERC)-funded PhD (NE/L501566/1).

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Contributions

E.H.D. and M.A. collected pebble lithology data and mapped positions of the gravel–sand transition. E.H.D. calculated gravel fluxes and proportions. M.A. devised the pebble abrasion model, which E.H.D. ran and analysed the results from. E.H.D., M.A. and H.D.S. designed the study and all discussed the results to shape this manuscript. E.H.D., M.A. and H.D.S wrote the manuscript. Figures were produced by E.H.D.

Corresponding author

Correspondence to Elizabeth H. Dingle.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks E. Garzanti, C. Paola and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Details of pebble lithologies documented on exposed gravel bars along trans-Himalayan rivers upstream of the gravel–sand transition.

Data in Fig. 3b represent an average of the sites downstream of the mountain front for each river. Note that Siwalik lithologies were found on bars sampled along the Kosi River, despite no Siwalik units being mapped in the catchment geology21; this is probably due to the coarse nature of the Himalayan scale geological map21, where small outcrops may have been omitted. Distances are relative to the mountain front, so negative distances are upstream of the mountain front.

Extended Data Figure 2 Sensitivity of gravel proportions to the position of the gravel–sand transition.

Gravel proportions were calculated for instances where the gravel–sand transition was 5 km further downstream and upstream of the mapped position to test the effect on the results presented in Fig. 2b; these changes are reflected by the increased length of error bars associated with each river, but the overall patterns remain unchanged. As in Fig. 2b, gravel percentage values are estimated by dividing the flux of gravel calculated based on fan geometry and location of the gravel–sand transition by the total sediment flux from (1) catchment-averaged 10Be derived erosion rates for trans-Himalayan catchments18, and (2) a range of possible catchment-averaged erosion rates for the foothill-fed catchments19. Foothill-fed catchments are shaded in grey. Red, blue and yellow data points correspond to maximum, average and minimum total sediment flux scenarios, respectively, with corresponding erosion rates (in mm yr−1) indicated next to data points for maximum and minimum flux scenarios for reference. Error bars reflect differences in accommodation space generated under maximum and minimum subsidence rates17.

Extended Data Figure 3 Schematic of gravel abrasion and sediment pulse delivery from the interior of the Himalayan mountains into the Ganga Plain.

Schematic comparison of the evolution of coarse sediment pulses generated in the Greater Himalayas and Siwalik Hills, as a result of earthquake-induced landsliding. The magnitude and extent of the pulses as they travel downstream is unknown, as is the timescales over which the pulses migrate27. a, As the sediment pulse is translated and dispersed downstream27, a combination of abrasion of weaker lithologies sourced in the Higher Himalayas and greater transport distances minimizes the gravel flux reaching the Ganga Plain, downstream of the mountain front. b, In contrast, stronger quartzite pebbles sourced from the Siwalik Hills undergo much less abrasion and, when combined with shorter transport distances, a larger gravel flux survives into the Ganga Plain when landsliding is focused closer to the mountain front. A large fraction of this gravel will probably remain trapped upstream of the gravel–sand transition, whereas more mobile sand and finer sediment (generated by the landslide inputs themselves and from the abrasion of coarser sediments) can be transported and deposited further downstream; where and when this finer sediment is deposited between the mountain front and the tip of the Bengal fan is less well understood. c, Where gravel flux downstream of the mountain front is enhanced, gravel aggradation could reduce channel capacity and enhance over-bank flooding. The extent of flooding is exacerbated by the low-relief topography that characterizes sedimentary basins downstream of large mountain ranges.

Extended Data Table 1 Subsidence and fan geometries used to calculate gravel flux
Extended Data Table 2 Subsidence and fan geometries used to calculate gravel flux
Extended Data Table 3 Sediment fluxes and gravel ratios

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Dingle, E., Attal, M. & Sinclair, H. Abrasion-set limits on Himalayan gravel flux. Nature 544, 471–474 (2017). https://doi.org/10.1038/nature22039

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