EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms (2017) Copyright © 2017 John Wiley & Sons, Ltd. Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.4258 Spatiotemporal variations of suspended sediment transport in the upstream and midstream of the Yarlung Tsangpo River (the upper Brahmaputra), China Xiaonan Shi,1,2,3 Fan Zhang,1,2,3* Xixi Lu,4 Zhaoyin Wang,5 Tongliang Gong,6 Guanxing Wang1,2 and Hongbo Zhang1,2 1 Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China 2 Key Laboratory of Alpine Ecology and Biodiversity, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China 3 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China 4 Department of Geography, National University of Singapore, Arts Link 1, Singapore, 117570 5 State Key Laboratory of Hydro-Science and Engineering, Tsinghua University, Beijing 100084, China 6 The flood control and drought relief headquarters office of Tibet Autonomous Region, Lhasa, Tibet 850000, China Received 15 June 2016; Revised 5 September 2017; Accepted 6 September 2017 *Correspondence to: Fan Zhang, Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China. E-mail: [email protected] ABSTRACT: The Yarlung Tsangpo River, which flows from west to east across the southern part of the Tibetan Plateau, is the longest river on the plateau and an important center for human habitation in Tibet. Suspended sediment in the river can be used as an important proxy for evaluating regional soil erosion and ecological and environmental conditions. However, sediment transport in the river is rarely reported due to data scarcity. Results from this study based on a daily dataset of 3 years from four main stream gaug- ing stations confirmed the existence of great spatiotemporal variability in suspended sediment transport in the Yarlung Tsangpo River, under interactions of monsoon climate and topographical variability. Temporally, sediment transport or deposition mainly occurred during the summer months from July to September, accounting for 79% to 93% of annual gross sediment load. This coincided with the rainy season from June to August that accounted for 51% to 80% of annual gross precipitation and the flood period from July to September that accounted for approximately 60% of annual gross discharge. The highest specific sediment yield of 177.6 t/km2/yr occurred in the upper midstream with the highest erosion intensity. The lower midstream was dominated by deposition, trapping approximately 40% of total sediment input from its upstream area. Sediment load transported to the midstream terminus was 10.43 Mt/yr with a basin average specific sediment yield of 54 t/km2/yr. Comparison with other plateau-originated rivers like the upper Yellow River, the upper Yangtze River, the upper Indus River, and the Mekong River indicated that sediment contribution from the studied area was very low. The results provided fundamental information for future studies on soil and water conservation and for the river basin management. Copyright © 2017 John Wiley & Sons, Ltd. KEYWORDS: suspended sediment transport; soil erosion; Yarlung Tsangpo River; Tibetan plateau Introduction Suetsugi, 2014). Suspended sediment is important for current erosion rate estimation (Jansson, 2002; Pelletier, 2012; Dean As an important process in hydrologic and ecological systems, et al., 2016), and plays an important role in the transfer of fluvial sediment has been identified as one of the most impor- most of the available nutrients and contaminants (Devesa- tant proxies in studies on soil erosion, channel evolution, hy- Rey et al., 2009). Over recent decades, there has been a per- draulic engineering, and aquatic ecology (Ritchie and sistent focus on the spatiotemporal patterns of sediment loads Schiebe, 2000; Walling, 2009; Hodge et al., 2011; Mueller in the global rivers, such as the Amazon River (Meade et al., and Pitlick, 2013, 2014; Bravard et al., 2014; Park and 1979; Richey et al., 1986), the Danube River (Mladenovic Latrubesse, 2014; Nearing et al., 2017). The sediment in a et al., 2013; Tóth and Bódis, 2015), the Yellow River (Fu, river system is composed of suspended and bed loads. 1989; Xu, 2004; Wang et al., 2007a), as well as cold-region Suspended sediment is predominant and commonly accounts rivers (Demildov et al., 1995; Glasser et al., 2004; Ollesch for approximately 90% of the total yields (Walling and Fang, et al., 2006; Singh et al., 2008; O’Farrell et al., 2009; Favaro 2003; Francke et al., 2008; Zhang et al., 2012; Heng and and Lamoureux, 2015), showing their dependence on factors X. SHI ET AL. such as climate change, geomorphologies, land use, and hu- the riverine sediment transport process in the region because of man activity. The Tibetan Plateau has one of the largest ice data scarcity, although it has been identified as the subject of masses on the Earth, referred to as the Asian ‘ice reservoir’ land surface hydrological processes of alpine region research (Qiu, 2013). It is the headwaters of many major Asian rivers, (Knight and Harrison, 2009). The gross sediment load output supporting billions of people in surrounding regions. These from the Yarlung Tsangpo River was roughly estimated as a rivers are also important carriers for sediment transported to small fraction (<10%) of the total load in the Ganges– the oceans. As estimated, around one-third of the global sed- Brahmaputra River (Wasson, 2003; Blöthe and Korup, 2013). iment loads to the oceans was generated from the large rivers Wang et al. (2015b) measured the depth of deposition layer originated from the Tibetan Plateau and its neighboring re- above bedrock at 29 cross-sections along the mainstream of gions (Milliman and Meade, 1983). Sediment transport re- the Yarlung Tsangpo River and estimated the total volume of gimes in the large rivers originated on the Tibetan plateau, sediment storage in the river valley as 518 billion m3, with de- such as the Yellow, the Yangtze, the Indus, and the Ganges– posits of mainly coarse gravel and sand. The present channel Brahmaputra Rivers, have received increasing attention be- was at an unstable state with continuous sediment deposits cause of the drastic changes (Lu, 2004). For example, the Yel- (Wang et al., 2015b). However, these arguments urgently need low River, once the river with the highest sediment load in the evidence in terms of riverine sediment data. Moreover, the spa- world (Shi et al., 2002), witnessed a continuous decrease of tial patterns and seasonal variation of suspended sediment sediment load since the 1950s by approximately 90% (Fu, transport are largely unknown in the Yarlung Tsangpo River. 1989; Xu, 2004; Wang et al., 2007a) as a result of regional cli- The primary objective of this study was to investigate spatial mate change and human activity in the Loess Plateau (Shi and temporal variations of suspended sediment in the Yarlung et al., 2002; Wang et al., 2010; Yu et al., 2013; Wang et al., Tsangpo River to understand sediment transport characteristics 2015a; Wang et al., 2016). The Yangtze River, ranked as the in terms of climatic and hydrological responses, sediment rat- 5th largest globally in terms of discharge and sediment load, ing curve analysis, and riverbed morphology and topography. also experienced a drastic reduction in sediment load (Yang The study was based on available data of daily discharge and et al., 2011; Dai and Lu, 2014). The upper river basin was re- suspended sediment concentration from four main stream ported to be the main sediment source for the Yangtze River gauging stations during the period 2007–2009. Results of the (Lu and Higgitt, 1999; Lu, 2004; Wang et al., 2007b), while study will aid in the identification of critical areas of erosion significant deposition occurred in the midstream and down- and determine features of sediment transport in the region, a stream (Chen et al., 2001; Yang et al., 2007a, 2007b; Wang foundation for future studies on soil and water conservation et al., 2007b). In the Mekong River, an extremely important in the Yarlung Tsangpo River basin. international river flowing across six Asian countries, the tem- poral variation of sediment load was not consistent with changes in the water discharge (Wang et al., 2011; Zhai Study Area et al., 2016), which was attributed to soil disturbance and sed- iment trapping due to dam construction (Wang et al., 2011). The Yarlung Tsangpo River originates from the GyimaYangzoin Wang et al. (2011) identified the largest sediment source area Glacier at an elevation of 5200 m a.s.l. (above sea level) on the above Chiang Saen and the largest sediment sink between northern slope of the Himalayan Mountains (Liu et al., 2014). It Luang Prabang and Nong Khai. The Indus River, as a result flows from west to east across the southern part of the Tibetan of the combined effect of alpine topography, meltwater sup- Plateau with a total length of 2057 km, an average gradient of ply, and the summer monsoon, transported a large volume 2.6‰, a mean elevation of 4621 m a.s.l and a basin area of of sediment (Nag and Phartiyal, 2015). In particular, the up- 240 480 km2 (from 80°120 E to 97°380E and from 27°26 N to stream of the Indus River in northern Pakistan had one of 28°540N) (Figure1) (Guan et al., 1984; Liu et al., 2007; Yao the highest rates of sediment transport reported (Meybeck, et al., 2010). The Yarlung Tsangpo River is the upper part of 1976; Ali and Boer, 2007). The Ganges–Brahmaputra River the Brahmaputra River which joins water from the Meghna was one of the most sediment-laden rivers in the world River before emptying into the Bay of Bengal.