Impacts of Dam Constructions on the Annual Distribution of Sediment in Lancang-Mekong River Basin

Annual distribution characteristic of river sediment concentration is closely related to climate change and anthropogenic activities and directly affected by the process of hydraulic resource development in the Lancang-Mekong River basin.Using monthly suspended sediment concentrations in 1987-2003 at the three hydrological stations,Jiuzhou,Yunjinghong and Chiang Saen stations,on the mainstream of Lancang-Mekong River,the uneven distributions coefficient,the concentrated rate and period,and the range of variation of annual distribution characteristics of sediment concentrations at the three stations were studied and then the responses of these characteristics to the progress of upstream dam constructions were analyzed.The research results show that:(1) Annual distribution of sediment concentration at Jiuzhou hydrological station agrees with the variation trend of local climate changes,its uneven distribution coefficients(AUDC) increased at the studied period and maintained the characteristics of natural river;AUDCs at Yunjinghong and Chiang Saen stations seemed to be affected by anthropological activities such as dam constructions rather than climate changes,however,sediments of two downstream stations responded differently to the dam constructions.AUDC at Yunjinghong station decreased from the periods of 1987-1992 to 1993-1996,then abruptly increased during 1997-2003;while AUDC at Chiang Saen took on a trend of moderate decreasing in the whole period.(2) Rates and distribution periods of sediment concentrations at Jiuzhou station almost kept stable during the studied period;but those at Yunjinghong and Chiang Saen changed evidently,their distribution periods delayed asynchronously during the dam construction processes(1987-1992,1997-2003),in the period of Manwan dam construction,distribution periods at Yunjinghong station delayed 5-6 days,while Chiang Saen delayed almost half a month,and in the Dachaoshan dam construction,delayed period at Yunjinghong station was longer than that at Chiang Saen.(3) Relative and absolute ranges of sediment variations at Jiuzhou station both increased against the time phases;those at Yunjinghong decreased,while relative ranges of sediment variation at Chiang Saen decreased in the earlier years then increased later,but absolute range was decreasing all the time.Different responses of annual distributions of sediment concentration at three hydrological stations to dam constructions indicate that their driving factors of sediment variation are absolutely different.These discoveries of annual distribution characteristics of sediment concentrations present new scientific evidences for assessing the impacts of dam constructions on upstream and downstream sediment change as well as its transboundary effects.