Basin-wide sediment trapping efficiency of emerging reservoirs along the Mekong

The Mekong Basin has remained relatively intact, but the current plans for rapid development in the hydropower sector may threaten the riverine ecosystems. Should all the plans be materialized in tributaries and mainstream, the cumulative active storage capacity of the reservoirs would increase more than tenfold from the present level to around 20% of the annual discharge of the Mekong (505 km(3)). In this study a protocol is developed to estimate the trapping efficiency (TE) of the existing and planned reservoirs in the Mekong Basin based on Brune's method. The existing reservoirs have a basin TE of 15-18% and should all the planned reservoirs be built, this will increase to 51-69%. However, due to the high heterogeneity of the specific sediment yield in different parts of the basin, the trapped sediment load (TSL) is predicted to be much higher. The existing and planned mainstream dams in the Chinese part of the river have the largest impact on the river sediment load (SL) as more than 60% of the basin SL originates from this stretch of the river. The three existing reservoirs in that part of the basin have potential to trap annually approximately 32-41 Mt of sediment. If the entire cascade of eight dams is constructed, TE will increase to 78-81%, and potentially 70-73 Mt, i.e. more than 50% of the total basin sediment load (similar to 140 Mt) will be trapped annually. (C) 2010 Elsevier B.V. All rights reserved.