Water Discharge and Total Suspended Sediment Dynamic in the Tonle Sap River at Prek Kdam
Prek Kdam station records flow and suspended sediment load in the Tonle Sap River, downstream from Tonle Sap Lake to the Mekong during the dry season, and reverse, upstream flow during high stages of the Mekong River (Figure 4). We used negative values of water discharge and sediment concentration to denote reverse flow from the Mekong River into the Tonle Sap Lake, and positive values for outflow from the Tonle Sap Lake towards Mekong River.
The number of outflowing and inflowing days toward Tonle Sap Lake from Mekong River through Tonle Sap River fluctuated from year to year. The reversal began in late May to early June, with water flowing from Mekong River upstream into Tonle Sap Lake for 70 to 157 days per year (average of 118 days), whereas flow downstream from Tonle Sap Lake to the Mekong River occurred for 209 to 295 days (average 247 days). 1995 and 2014 were years with the fewest days of reverse flow, 73 and 70 days, respectively.
The peak reverse flow in Prek Kdam generally occurred in July and August, before the peak discharge of Mekong River, in August and September at Chroy Changvar station. Peak outflow from the lake to Mekong River mainly took place a few months later, during the outflows from October to December. During the observed periods, the maximum reverse flow into the lake was 10,679 m3/s, while the maximum outflow from the lake was 10,104 m3/s.
Suspended sediment concentrations at Prek Kdam were governed by distinct flow directions (Table 5) . TSS concentrations in the Tonle Sap River for outflow from Tonle Sap to the Mekong (October to March) averaged 41 mg/l, while TSS concentrations during reverse flows from the Mekong River towards Tonle Sap Lake (May to September) averaged 74 mg/l. Thus, TSS concentrations in reverse flow from Mekong River to the Lake were nearly twice the concentrations in the outflow from Tonle Sap Lake, implying a net transfer of sediment into the Lake. The sediment rating curves at Prek Kdam show distinct relations when plotted separately for outflow and reverse-flow periods (Figure 5 ), with TSS concentrations lower in outflows from Tonle Sap Lake to the Mekong (Figure 5.a) than in reverse flows from the Mekong river to Tonle Sap Lake (Figure 5.b) , with all parameters having only a weak relationship in linearity with discharge (R2< 0.1) in both flow periods.