Isotopic investigation of river water mixing around the confluence of the Tonle Sap and Mekong rivers

The Great Lake (Tonle Sap), located in mid-western Cambodia, is the largest lake in southeast Asia and joins the Mekong through the Tonle Sap River. During the annual monsoon peak from about the end of May, the flow direction in the Tonle Sap reverses and river water from the Mekong flows to the Great Lake. When the water level of the Mekong decreases sufficiently, usually in October, the Tonle Sap again flows in its normal direction from the Great Lake to the Mekong. This study quantitatively investigated interactions between the Tonle Sap River and the Mekong River using the stable isotope ratios of oxygen (delta O-18) in waters from both rivers, and estimated the contribution rate of Tonle Sap River water to Mekong River water during the peak flow period of the Tonle Sap River. During this period, the Tonle Sap River contributed approximately 70-97% along the right bank and 0-5% in the middle and left banks of the Mekong. We also observed delta O-18 of the Mekong River along a longitudinal section on the right bank side, where the contribution rate of the Tonle Sap River water remained high (80%) even 21 km downstream from the confluence. This study clearly demonstrates that we can determine the mixing rate of these rivers at each point after they converge by using stable oxygen isotopes as tracers, when the Tonle Sap flows in its normal direction. Copyright (C) 2008 John Wiley & Sons, Ltd.