Abstract
Glacial and proglacial erosion are important sediment sources in a river basin. The retreat of many glaciers on the Tibetan Plateau has important implications on the supply of fresh water and sediment dynamics for downstream river basins. Despite the importance of water and sediment dynamics at these catchments, existing quantification of suspended sediment fluxes from glacial catchments on the Tibetan Plateau is limited due to poor accessibility and challenging environments. This study presents the results of in-situ investigations of water discharge and suspended sediment fluxes from the Ming Yong glacial catchment in Yunnan, Southwest China, between August 2013 and July 2017. The results show that the variation in water discharge and suspended sediment was highly seasonal. The variation of average suspended sediment concentration was large – 69±45 mg/L; 119±104 mg/L; 94±97 mg/L in 2013, 2015, 2016, respectively. We estimate that the sediment yield from Ming Yong catchment was highly variable ranging from 1104 t/km2/year in 2013to 2281 t/ km2/year in 2016, with 65-78% of the total annual sediment load occurring during summer (June to August). These annual variations in the sediment yield can be attributed largely to precipitation patterns, or otherwise, extreme melting events. This study has provided a benchmark dataset that can be used for further works that investigate the impact of climate change on sediment dynamics in glacierized catchments in the Tibetan Plateau. Subsequently, the study let us better understand the increasing sediment supply to the Upper Mekong River from glacierized catchments.
Introduction
The meltwater from glaciers on the Tibetan Plateau feeds many large Asian rivers, for example, Indus, Ganges, Brahmaputra, Salween, Mekong, Tarim, Syr Darya (Li et al., 2021). Millions of people in the region depend on the rivers for their livelihoods. With an average elevation exceeding 3,000m, the Tibetan Plateau contains around 36,800 glaciers occupying a total area of ~50,000 km2(Yao et al., 2012). These physical conditions caused the Tibetan Plateau to be more susceptible to solar insolation and amplified its sensitivity to global warming (Pepin et al., 2015). Indeed, Yao et al. (2012) found that glacial retreat on the Tibetan Plateau has intensified since the 1990s.
Glaciers are powerful agents of erosion through denudation mechanisms, generating large amounts of glacial debris in the process. Thus, sediment yields from glacial basins are higher than the global average (Hallet et al., 1996). Suspended sediment fluxes from proglacial areas, accompanied by intensified meltwater and increased sediment availability, have increased in response to accelerated glacier retreat ( Li et al., 2021; Overeem et al., 2017). In glacier regions, the roles of sediment are double-edged – while it is essential in the maintenance of riverine ecology, morphology, agriculture, and fisheries by providing nutrients and materials (Walling, 2006),sediments absorb toxic chemicals such as mercury. Therefore, high concentrations of sediment degrade water quality, change aquatic habitats, and cause civil engineering problems for dams and river transportation (Li et al., 2021). Furthermore, given the context of climate change, understanding sediment dynamics from glacial catchments, such asthe Tibetan Plateau, is important for land resource management and planning (Li et al., 2021).
Sediment transport vary spatially along a proglacial river and is characterized by strong seasonal and diurnal variabilities (Beylich et al., 2017). Proglacial sediment is predominantly produced by glacial movements and transported by glacial meltwater, from the melting of ice on the ice surface (supraglacial), at the bed (subglacial), or within the glacier (englacial) (Heckmann et al.,2016). Heat required to melt the ice can be supplied by solar radiation or geothermal heat beneath the glacier (Bennett & Glasser, 2011). Therefore, proglacial discharge hydrographs follow the temporal changes in solar incidence on annual and diurnal timescales (Miles et al., 2020). Also. the rates at which meltwater and sediment are drained depend on the type of pathways and the channel size (Carrivick & Tweed, 2021).
Previous studies on the quantification of suspended sediment load on the Tibetan Plateau are mostly derived from hydrological gauging stations that are located far away from the glaciers (Li et al., 2021; Shi et al., 2018). Also, field measurements of suspended sediment fluxes from proglacial rivers in the Tibetan Plateau are very limited and spatially scattered. For example, Kumar et al. (2002) measured discharge and suspended sediment in the meltwater of Gangotri Glacier in Garhwal Himalaya, India; Kireet et al. (2012) analyzed the spatio-temporal trends of suspended sediment flux along the Sutlej River and its main tributaries in western Himalaya; Srivastava et al. (2014) measured discharge and suspended sediment load at Dunagiri Glacier basin located in Garhwal Himalaya between 1984 and 1989. Nearer to Southeast Asia, studies that examined sediment loads in the Mekong basin have reported a lack of sediment data from the Upper Mekong located within China (Lu and Siew, 2006; Walling, 2008; Wang et al., 2011). Particularly, the glaciated catchments there are poorly studied and, to our best knowledge, there has not been any study done to quantify the sediment load there.
Consequently, this study focuses on Ming Yong Glacier in Yunnan Province, which islocated in the southeast region on the Tibetan Plateau. The glacier was found to have retreated by 190m between 1998 and 2004, with 110m of the retreat occurring between 2002 and 2004 (Baker & Moseley, 2007). Against this reduction, knowledge about sediment delivery from the glacial catchments in the Upper Mekong basin is ever more important in the understanding of sediment delivery dynamics for the whole Mekong basin (Lu et al., 2014). Therefore, this study aims to: (1) quantify the sediment yield and sediment load from the Ming Yong glacial catchment; (2) analyze the temporal variability of suspended sediment flux from Ming Yong glacial catchment; and (3) discuss the potential drivers and implications of changing sediment yield for the Upper Mekong basin in the context of climate change.