Sediment exchange between Tonle Sap River and Mekong mainstream
Our analysis shows that from 1995 to 2000, the Tonle Sap contributed more sediment load to Mekong River than was deposited in the lake, on the average 0.65 Mt annually, but the rate decreased, and then since 2001, an average net 1.35±0.7 Mt of sediment has been deposited in the lake annually. This contrasts with estimates of Kummu et al. (2008) that over the period 1997-2003, the Tonle Sap Lake system received an average of 5.1 Mt/year of sediment from the Mekong by reverse flow and contributed 1.4 Mt/year to the Mekong via outflow, for a net deposition of 3.7 Mt/y. XiXi Lu, Kummu, and Oeurng (2014) calculated 6.3 Mt/year for the mean sediment inflow into the lake from the Mekong mainstream and 7 Mt/year in the outflow during a three-year observation period (2008–2010), showing a net deposition within Tonle Sap Lake, more consistent with our results. Our estimate of mean annual sediment load in reverse flow from the Mekong River mainstem to Tonle Sap lake of 4.2 Mt/year is lower than the 5.1 Mt reported by Kummu et al. (2008) and 6.3 Mt calculated by XiXi Lu, Kummu, and Oeurng (2014). The mean annual sediment outflow from the Tonle Sap lake calculated in this study (3.1 Mt /year) is higher than mean value of 1.4 Mt/year reported by Kummu et al. (2008) but lower than the mean value of 7 Mt/year calculated by XiXi Lu, Kummu, and Oeurng (2014).
We used water quality data obtaining from MOWRAM under the MRC water quality program. (As noted above, these were not depth-integrated samples, so the accuracy of suspended load estimates from these depends on how well-mixed are sediment concentrations vertically and horizontally.). Kummu et al. (2008) analysed data from the same database but only from 1997-2003, whereas our study used data from hydrological year 1995-2017. Instead of data from Prek Kdam station, XiXi Lu, Kummu, and Oeurng (2014) used data from the Phnom Penh Port from their depth-integrated samples, located 30km from the Chatukmuk confluence. The different location could be significant, as in between these two stations there is a floodplain connected to the Mekong mainstem (upstream from Chatumuk confluence) such that overbank flows from the Mekong can enter the Tonle Sap River and thus might contribute sediment to the Tonle Sap River during high flows.
An assessment of the variability of water discharge and sediment loads of Mekong River presented in this study helps clarify the exchange annual discharge and sediment load toward the Mekong Delta. Tonle Sap Lake provided sediment to the Mekong system and Delta annually 0.65±0.6 Mt from 1995 to 2000, but since 2001 Tonle Sap Lake has become a sediment sink for about 1.35±0.7 Mt annually, thereby reducing the annual sediment transport to the Mekong delta. This reduction in sediment supply compounds the threat to the Mekong delta from accelerated subsidence and sea level rise (Syvitski & Higgins, 2012) (Pokhrel et al., 2018). Decreased sediment loads and alterated sediment transport processes that drive river and delta morph-dynamics will impact many livelihoods in the basin that depend on ecosystem services (G. M. Kondolf et al., 2018).
Conclusion
The study assessed temporal variability of sediment loads in Lower Mekong River in Cambodia and the sediment linkage between Tonle Sap Lake and Mekong River from 1993 to 2017. The annual water discharge in Lower Mekong River was 404 Km3 at Kratie and 368 Km3 at Chroy Changvar near the Chatumuk confluence. We estimated sediment load in the main Mekong River averaged 72± 38 Mt at Kratie and 78±22 Mt at Chroy Changvar from 1993-2018, lower than previous studies for the period before the 2000s, i.e., prior to dam construction on the Mekong mainstem and tributaries. Over the study period, the Lower Mekong River showed a significant decrease in sediment load, consistent with trends documented in other major rivers in the region and globally. Two factors controlling the sediment linkage between the Mekong mainstem and Tonle Sap Lake are concentrations of the suspended sediment and annual water discharge in reverse flows from the river and outflows from the lake through the Tonle Sap River. The concentration of suspended sediment in reverse flows from the Mekong River was twice that of outflows from the Lake. However, net inflow to the lake via reverse flow was 36 km3, while outflow to the Mekong was 68 km3. Tonle Sap Lake provided 0.65±0.6 Mt of sediment to the Mekong annually from 1995 to 2000, but was a sediment sink for an average of 1.35±0.7 Mt annually from 2001 onwards.
This study helps clarify the sediment exchange between Mekong River and Tonle Sap Lake and reveals an important change since 2001, with implications for the sustainability of the Delta.