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.