Berkeley, CA 94720, USA
Abstract
The Mekong River, one of the world’s great rivers, is facing the disruption of its sediment balance with anthropic reductions in its sediment load and resultant impacts on nutrient fluxes, aquatic ecology and evolution of its river channel, floodplain and delta. Using long-term monitoring data from 1993-2018, we estimated the temporal variability of sediment loads in Tonle Sap and Lower Mekong Rivers in Cambodia, assessing the sediment linkage between Tonle Sap Lake and Mekong River, which are connected by a seasonally reversing flow through the Tonle Sap River. We used data from three monitoring stations established in Cambodia in 1993, from the Mekong at Kratie (upstream) downstream to the Mekong at Chroy Changvar (just upstream of the Tonle Sap confluence), and the Tonle Sap River at Prek Kdam (about 40 km upstream of the Mekong confluence). We estimated the annual sediment in the main Mekong River was 72±38 Mt/yr at Kratie and 78±22 Mt/yr at Chroy Changvar from 1993-2018. Our calculated sediment load for the Lower Mekong River is lower than reported in older studies (prior to the 2000s), which is consistent with sediment trapping by dams on Upper Mekong mainstream and major tributaries built since 1993, and consistent with other recent estimates of sediment load on the Lower Mekong. Our analysis of water discharge and sediment concentration indicates that Tonle Sap Lake provided 0.65±0.6 Mt of sediment annually to the Lower Mekong River from 1995 to 2000. However, since 2001, Tonle Sap Lake has become a sink for sediment, accumulating an average of 1.35±0.7 Mt annually. Net storage of sediment in Tonle Sap Lake reduces the annual sediment transport to the Mekong delta, further compounding the effects of sediment delivery to the Delta resulting from upstream dam construction and instream sand mining.
Keywords: Temporal variability, Sediment load; Mekong River; Tonle Sap Lake
* Corresponding Author: Ty SOK (sokty@itc.edu.kh)
Chantha OEURNG (chantha@itc.edu.kh)
Introduction
Sediment erosion, transport, and deposition within river basins are the main surficial processes that have influenced the geomorphology on river channels, floodplains, and deltas (Peng, Chen, & Dong, 2010). In recent decades it has been increasingly realized that river dynamic conditions are greatly influenced by human activities such as hydropower dam development which have led to considerable changes of water discharge and sediment load (Vörösmarty, Meybeck, Fekete, Sharma, Green, Syvitski, et al., 2003). The magnitude of the suspended sediment load transported by a river has important implications both for the natural functioning of the river system, for instance, through its influence on channel morphology, water quality and aquatic ecosystems, as well as habitats supported by the river, and for human exploitation of the river system (D. E. Walling, 2009). Analysis of changes in runoff and sediment loads of 145 rivers worldwide showed decreased sediment loads in most rivers (D. Walling, Fang, & change, 2003; D. J. G. Walling, 2006). Cheng, Jueyi, and Zhao-Yin (2008) reported that all the large rivers in China discharged less sediment into the sea in recent years than the long term average. For major rivers in Southern China, such as the Yangtze and Qiantang rivers, and Dongjiang (East), Xijiang (West) and Beijiang (North) rivers in the Pearl River Basin, the annual sediment loads entering the sea in the past 10 years are only about 60%–80% of their multi-year mean values, while their annual runoff has remained relatively stable, based on annual runoff and sediment data to 2005 (C Liu, Wang, & Sui, 2007).
The Mekong River flows 4,800 km from its headwaters on the Tibet-Qinghai plateau before it enters the South China Sea in Vietnam, with a mean annual water discharge of 470 km3 (XX Lu & Siew, 2005). The river’s uppermost 2,000 km, known also as the Lancang, flows through China, draining an area of 195,000 km2. The lower basin covers an area of 600,000 km2 in Myanmar, Thailand, Laos, Cambodia, and Vietnam. The Mekong River region is experiencing extensive land surface disturbance such as forest clearing, arable land expansion, reservoir construction, and water diversion, as a result of rapid population growth and expanding urbanization (XX Lu & Siew, 2005). Agricultural, ecological, and fish productivity in the lower Mekong, particularly in the Tonle Sap lake in Cambodia and the Mekong Delta in Vietnam, supporting over 60 M people, are attributed to the seasonal delivery of water, sediments, and nutrients (Arias et al., 2014a; Kummu et al., 2008; Lamberts, 2006). Changes in sediment and nutrient are of particular concern in tropical regions undergoing rapid development, which has impacted on delta shrink and biodiversity. The Mekong is facing the disruption of its nutrient balance as significant increases to nutrient inputs to surface water is expected in the twenty-first century due to increases in agricultural production and infrastructure development (Galloway et al., 2004; Liljeström, Kummu, & Varis, 2012; M. R. C. MRC, 2003).
The possibility of changes in the sediment load of the Lancang-Mekong River has attracted attention because changes in the sediment load of a river can have profound impacts on river channel evolution, nutrient fluxes, aquatic ecology and delta erosion and sedimentation (Cheng Liu, He, Des Walling, & Wang, 2013). Hydropower development is altering sediment loads in the Mekong, first from dams in upper basin (Kummu and Varis (2007). Analysis of changes in annual sediment loads at 7 stations located along the river from Gajiu (upper Mekong) to Khong Chiam (Lower Mekong) was done by Cheng Liu et al. (2013), and estimates of annual sediment loads at the five mainstream stations on the Lower Mekong (Chiang Saen, Luang Prabang, Nong Khai, Mukdahan and Khong Chiam) from 1985-2000 by J. J. Wang, Lu, and Kummu (2011) and nutrient loads for these stations from 1985–2011by Li and Bush (2015) show decreases, but covered the river only down to Pakse in Loas. The lowermost reach of the Mekong, through Cambodia and into the Delta, including 3S river system (Sekong, Sesan and Srepok), the biggest sub-basins of the Mekong river, and the Tonle Sap Lake system have received more limited study. A better understanding of the sediment linkage between the Mekong mainstream and Tonle Sap Lake is needed because sediment input from the Mekong is crucial for the Tonle Sap’s ecosystem functions (Arias et al., 2014). The sediment exchange between Tonle Sap Lake and Mekong River was previously estimated by Kummu, Penny, Sarkkula, and Koponen (2008) over the period 1997–2003 and by XiXi Lu, Kummu, and Oeurng (2014) during a three-year observation period 2008–2010. XiXi Lu, Kummu, and Oeurng (2014) found that the sediment outflow was higher than the inflow; whereas Kummu et al. (2008) previously reported inflow sediment load towards the lake was higher than that of the outflow sediment.
However, a more complete study of the long-term sediment and nutrient transport between the Mekong mainstream and the Tonle Sap River has been needed to better understand the functioning of the highly productive Tonle Sap Lake system and to estimate sediment delivery to the Mekong Delta. Hence, the objectives of this study is to estimate the temporal variability of sediment loads in Tonle Sap and Lower Mekong Rivers in Cambodia, and to assess the sediment linkage between Tonle Sap Lake and Mekong River.
Materials and Methods