Environmental tradeoffs of dams in the Lower Mekong Basin

In the Lower Mekong River, constructing and planning hydropower dams has historically taken precedence over analyzing their environmental effects. Hydropower development provides energy for developing countries, but also threatens biodiversity, ecosystems, food security, and an unparalleled freshwater fishery. Forty two dams have been built, are under construction, or are planned in the Sekong, Sesan, and Srepok Rivers (3S Basin), with eleven additional dams proposed or in construction on the mainstem Lower Mekong River. The 3S basin is 10% of the Mekong watershed, but supports nearly 40% of the fish species of the Lower Mekong Basin, species that move between the 3S Basin, through the mainstem Mekong River and rear downstream in Tonle Sap Lake. We completed a meta-analysis of environmental tradeoffs of existing and planned hydropower dams in the 3S and Lower Mekong Basin using 45 papers and reports written over the past two decades. More than 50% of studies evaluated hydropower tradeoffs with streamflow alteration metrics and over 30% of papers quantified sediment tradeoffs. Freshwater fish production, non-fish organisms, habitat area and connectivity, and trophic ecology objectives were infrequently included. We visualized environmental migratory biomass change, inundated area, mean trapped sediment load, and installed hydropower capacity tradeoffs when authors shared models and data, and highlighted dams with considerable environmental tradeoffs (e.g., Lower Sesan 2 and Sambor Dams) and dams that are less environmentally harmful (e.g., Xepian-Xenamnoy Dam). Spatial optimization of dam locations was a common approach for evaluating environmental tradeoffs, and assumes that future dam planning and development are not too far along to be informed by portfolio studies. A less common approach assumes specific dams will be constructed, and evaluated tradeoffs among hydropower and environmental objectives by changing dam design and operations. Unless streamflow limits natural systems, environmental objectives could be improved by considering organism, habitat, and ecosystem requirements beyond streamflow metrics.