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.