Hydrol. Earth Syst. Sci., 16, 2193–2217, 2012 www.hydrol-earth-syst-sci.net/16/2193/2012/ Hydrology and doi:10.5194/hess-16-2193-2012 Earth System © Author(s) 2012. CC Attribution 3.0 License. Sciences Climatic and geologic controls on suspended sediment flux in the Sutlej River Valley, western Himalaya H. Wulf1,*, B. Bookhagen2, and D. Scherler1 1Department of Earth and Environmental Science, Potsdam University, Potsdam, Germany 2Department of Geography, University of California, Santa Barbara, USA *now at: Remote Sensing Section, Helmholtz Centre Potsdam, GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany Correspondence to: H. Wulf ([email protected]) Received: 19 December 2011 – Published in Hydrol. Earth Syst. Sci. Discuss.: 11 January 2012 Revised: 8 June 2012 – Accepted: 22 June 2012 – Published: 20 July 2012 Abstract. The sediment flux through Himalayan rivers di- sediments in the high-elevation sector that can easily be mo- rectly impacts water quality and is important for sustaining bilized by hydrometeorological events and higher glacial- agriculture as well as maintaining drinking-water and hy- meltwater contributions. In future climate change scenarios, dropower generation. Despite the recent increase in demand including continuous glacial retreat and more frequent mon- for these resources, little is known about the triggers and soonal rainstorms across the Himalaya, we expect an in- sources of extreme sediment flux events, which lower wa- crease in peak SSC events, which will decrease the water ter quality and account for extensive hydropower reservoir quality and impact hydropower generation. filling and turbine abrasion. Here, we present a comprehen- sive analysis of the spatiotemporal trends in suspended sedi- ment flux based on daily data during the past decade (2001– 2009) from four sites along the Sutlej River and from four 1 Introduction of its main tributaries. In conjunction with satellite data de- picting rainfall and snow cover, air temperature and earth- Pronounced erosion in the Himalaya delivers large amounts quake records, and field observations, we infer climatic and of sediment to the Indus and the Ganges-Brahmaputra River geologic controls of peak suspended sediment concentration systems, which build up the world’s two largest submarine (SSC) events. Our study identifies three key findings: First, fans in the Arabian Sea (up to 10-km thickness) (Clift et al., peak SSC events (≥ 99th SSC percentile) coincide frequently 2001) and the Bay of Bengal (up to 16.5-km thickness) (Cur- (57–80 %) with heavy rainstorms and account for about 30 % ray et al., 2003), respectively. The sediment loads of these −1 −1 of the suspended sediment flux in the semi-arid to arid inte- rivers (Indus: 250 Mt yr , Ganges: 520 Mt yr , Brahma- −1 rior of the orogen. Second, we observe an increase of sus- putra: 540 Mt yr ) rank among the highest in the world pended sediment flux from the Tibetan Plateau to the Hi- and contribute ∼ 10 % to the global sediments reaching the malayan Front at mean annual timescales. This sediment-flux oceans (Milliman and Syvitski, 1992). Knowledge of the gradient suggests that averaged, modern erosion in the west- magnitude and distribution of orogenic erosion rates as well ern Himalaya is most pronounced at frontal regions, which as the operating processes is crucial for understanding how are characterized by high monsoonal rainfall and thick soil these landscapes evolve (Molnar and England, 1990; Small cover. Third, in seven of eight catchments, we find an anti- and Anderson, 1995) and how erosion might affect active tec- clockwise hysteresis loop of annual sediment flux variations tonics (Burbank et al., 1996; Clift et al., 2008; Thiede et al., with respect to river discharge, which appears to be related 2004, 2009; Wobus et al., 2005) and global climatic changes to enhanced glacial sediment evacuation during late summer. (Raymo et al., 1988; Raymo and Ruddiman, 1992). Fur- Our analysis emphasizes the importance of unconsolidated thermore, quantifying the spatiotemporal patterns and vari- ation of fluvial sediment flux is important, because it affects Published by Copernicus Publications on behalf of the European Geosciences Union. 2194 H. Wulf et al.: Climatic controls on suspended sediment flux in the Sutlej River Valley the lifetime of hydropower reservoirs and abrasion of hy- in the western Himalaya experience reduced snow cover dropower turbines (e.g. Singh et al., 2003). (Shekhar et al., 2010) as more precipitation falls in the form High topographic relief, steep river profiles, and elevated of rain. Therefore, recent climate change is likely to en- stream power all indicate high erosion rates throughout the hance surface erosion processes, especially in glacial and Himalaya (Finlayson et al., 2002; Vance et al., 2003). Partic- periglacial regions. ularly the eastern and western syntaxes are areas of high ex- In this study, we analyze daily river discharge and sus- humation and erosion (1–10 mm yr−1) (Burbank et al., 1996; pended sediment concentration (SSC) data from the Sutlej Burg et al., 1998; Finnegan et al., 2008; Stewart et al., 2008; River Valley in the western Himalaya to study the sediment Zeitler et al., 2001). In contrast, erosion rates on the oro- flux (i.e. discharge multiplied by SSC) characteristics in dif- graphically shielded Tibetan Plateau are significantly lower ferent geologic and climatic regions. We compare the sed- (< 0.03 mm yr−1), due to lower rainfall amounts and lower iment flux data from four sites along the main stem of the topographic relief (Lal et al., 2003). However, several stud- Sutlej and from four of its largest tributaries with remotely ies suggest that, during active monsoon phases, strong con- sensed rainfall and snow cover data, as well as air tempera- vective cells can migrate across the orographic barrier and ture and earthquake records to investigate the climatic and result in heavy rainfall events, which can mobilize enormous geologic controls on low-frequency, high-magnitude sedi- amounts of sediment in the orogen’s interior (e.g. Bookha- ment discharges. Previous research shows that such peak gen, 2010; Bookhagen et al., 2005; Craddock et al., 2007; events often account for a large fraction of the sediment bud- Wulf et al., 2010). Overall, the influence of monsoonal pre- get (e.g. Barnard et al., 2001; Bookhagen et al., 2005; Kirch- cipitation versus tectonic forcing on Himalayan landscape ner et al., 2001; Wulf et al., 2010). In a final step, we compare evolution remains debated throughout different timescales the new data with published sediment flux data from across and orogenic compartments (e.g. Burbank et al., 2003; Clift the Himalaya to identify spatial patterns and first-order con- et al., 2008; Galy and France-Lanord, 2001; Hodges et al., trols on sediment transport. 2004; Thiede et al., 2004, 2009). Long-term (> 103 yr) rates of erosion and models of land- scape evolution are typically based on thermochronologi- 2 Geographic, climatic, and geologic setting cal (e.g. Reiners et al., 2005) and cosmogenic nuclide data 2.1 Geographic setting (e.g. Bierman, 1994; Bookhagen and Strecker, 2012; von Blanckenburg, 2005), but these data do not distinguish be- The Sutlej River is the largest tributary of the Indus River tween different erosion processes and their variability. More and drains the third largest catchment area in the Himalaya direct measurements of fluvial sediment yields, spanning (ca. 55 000 km2 above 500 m a.s.l. – above sea level). Ap- years to decades, can be inferred from sediment accumula- proximately two-thirds of this area is located in China and tion rates in reservoirs (sediment trapping), or from measure- drains the Zhada Basin (cf. Fig. 1, Sutlej River subcatch- ments of suspended sediment and bedload fluxes in rivers ment number 5), which stretches NW–SE between the south- (sediment gauging) (Meade, 1988; Wulf et al., 2010). Al- ern edge of the Tibetan Plateau and the Mount Kailash though fluvial sediment measurements do not reliably record Range. To the west, the Indian part of the Sutlej Valley cov- low-frequency, high-intensity events and rarely include the ers a wide range of elevations between the Indo-Gangetic bedload fraction, they provide valuable insights into the be- Plains (400 m a.s.l) and the Himalayan Crest (6400 m a.s.l.) havior of rivers and their coupling to tectonics, weather and (Fig. 1). The catchment-average altitude is 4400 m a.s.l., climate (e.g. Wolman and Miller, 1960). This coupling be- and more than 80 % of the catchment area is located at tween climate and rivers also elucidates the impact of cli- > 4000 m a.s.l. with virtually no vegetation cover (Fig. 1a). mate change on surface erosion and fluvial sediment flux, The lower part of the catchment area (< 4000 m a.s.l.) is because increasing temperatures cause pronounced environ- located at the monsoon-impacted southern front of the Hi- mental changes in the Himalayan region (IPCC, 2007). malaya, where vegetation is lush and dense. Therefore, the In the western Himalaya, recent increases in air temper- primary land cover in the Sutlej Valley is bare ground atures (Shekhar et al., 2010) are likely causing the retreat (81.2 %), followed by trees and shrubs (7.2 %), cultivated ar- −1 of most glaciers (ca. 20–50 m yr ) over the past decades eas (6.8 %), glaciers (3.7 %), and lakes (1.1 %) (FAO, 2009). (Bhambri and Bolch, 2009; Scherler et al., 2011a). Retreat- Developed soils cover only a small fraction (< 15 %), mostly ing glaciers expose unstable paraglacial landscapes, which in the lower part of the Sutlej Valley. Glacial cover is partic- are highly susceptible to erosion processes driven by glacial ularly dense at the Himalayan Crest, where snowfall is high- runoff and rainfall (e.g. Meigs et al., 2006). Furthermore, the est (e.g.