The role of mega dams in reducing sediment fluxes: A case study of large Asian rivers

In order to sustain the ever growing population and to meet water and energy requirements of the rapidly growing economies, most of the large rivers draining through East, Southern and Southeast (ESSE) Asian region have been regulated all along their courses, over the past few decades. For instance, ESSE Asian countries (China, Taiwan, Vietnam, Myanmar, Thailand, India, Pakistan and Bangladesh) host about 250 mega dams and several tens of thousands of large and small reservoirs. The present study provides a revised estimate on annual suspended sediment fluxes of the large rivers draining through ESSE region, including the latest data of the Indian peninsula rivers. In the last 50 years, the combined annual sediment flux of the large Chinese rivers has been reduced from 1800 million tons (Mt) to about 370 Mt. We estimate that at present the Indian peninsular rivers collectively transport about 83 Mt of sediment annually. The Ganga–Brahmaputra and the Indus, contribute 850 and 13 Mt of sediments, respectively to the oceans. Our revised estimates suggest that at present the large rivers of ESSE region, collectively delivering ∼2150 Mt of sediment annually to the oceans. We show that at decadal scale, decline in sediment fluxes of the large Asian rivers are proportional to the number of mega dams present in the respective catchments. We also demonstrate that storage of sediment-laden water of major flood events (major-event), led to huge sediment trapping behind mega dams. Thus, ongoing and planned dam constructions activities across ESSE Asia may further reduce the annual sediment fluxes.