Sedimentation in the Manwan reservoir in the Upper Mekong and its downstream impacts

Dam construction on upper streams retains sediment and causes a series of effects downstream. China planned eight dams on the main stream of the Upper Mekong (Lancang) River, of which the Manwan and the Dachaoshan dams were completed and put into use in 1993 and 2003. From 1993, when the Manwan Dam started to store water, to 2003, the annual mean sediment trapped by the reservoir is estimated as 26.9–28.5×106 t/a, or 295.9–313.5×106 t over the 11 years. The reservoir capacity loss caused by the annual sediment trapped is around 20.7–21.9×106 m3/a, or 227.6–241.2×106 m3 over the 11 years, equivalent to 21.5–22.8% of the total storage capacity of the Manwan reservoir.Sediment trapped by the Manwan reservoir produced different effects on the sediment variations downstream. Suspended sediment concentration (SSC) and sediment discharge at Gajiu station (2 km downstream of the reservoir) was affected most significantly, but Yunjinghong station (401 km further downstream) saw lesser effects. Further downstream at Chiang Saen (746 km distant), SSC responded to the reservoir sedimentation in a different way from the Yunjinghong station, with an obvious decline at the beginning of the reservoir impoundment but started to recover since 1997. This was corroborated by the result of a Granger causality test on the monthly SSC time series at Yunjinghong and Chiang Saen, indicating that there was no evident causality between the sediment series of the two stations.