Dam choices: analyses for multiple needs.

E nvironmentalists often go to great lengths to block the construction of new dams because dams interfere with fish migration, inundate terrestrial habitats, and interrupt natural flows that are necessary for critical ecosystem processes. However, few environmental issues are so simplistically black and white, and dams are no exception. Dams can be an important source of clean energy and for poor nations such as Laos or Cambodia, hydropower is an export that can be a foundation for poverty alleviation. The right question is not how to prevent construction of any new dams, but rather what the optimal portfolio of dams is for meeting our energy and fisheries needs while also securing as much biodiversity insurance as possible for our changing world. Ziv et al. (1) from Princeton University and the World Fish Center in Cambodia ask that right question. Specifically, they combine an ingenious model of fish migration in the mainstem Mekong River and its tributaries with scenarios of new dam construction to identify optimal dam deployments throughout the Mekong tributaries (1). The Mekong River is one of the world’s last great rivers to remain largely undammed. However, this will not be true for long, as China launches eight hydropower projects in the upper Mekong where there is a lot of energy to be tapped as the river drops 4,000 m in elevation from its headwaters in Tibet to the China– Laos border (2, 3). Even more troubling to environmentalists are the dams being considered for the lower Mekong tributaries and mainstem. At stake are some of the world’s most productive inland fisheries—fisheries that depend on migratory fish species, including some spectacularly unique species as the migratory goonch catfish (Bagarius yarrelli), which can reach 0.5 m in length and 65 kg (2). To date quantitative analyses have focused either on the potential damage to species and fisheries from dams or on the energy and economic yields of hydropower. Although everyone admits there are trade-offs, formal analyses of those trade-offs have been lacking. This is where Ziv et al. step in. They frame the question as an optimization problem, where one seeks to maximize the migratory fish biomass while meeting some specified energy demand. This is a numerically rich problem because there are 11 possible mainstem dams and 27 possible tributary dams whose fates are yet undetermined. Considering just the tributary dams, there are 2 possible permutations of dams for which one can calculate energy yield and fish reductions. One of the big surprises from their analyses is that although most attention has been given to dams proposed for the mainstem of the lower Mekong, in fact the construction of all of the tributary dams would produce less energy and reduce fish biomass and diversity far more than the construction of six upper mainstem dams. Whenever environmentalists scrutinize hydropower projects, they invariably focus on mainstem dams because dams on the mainstem block a large portion of a river Fig. 1. Proposed hydropower dam projects in South America compiled from each country’s ministry of energy and/or planning agency between 2009 and 2011. The original data come from both online databases and official energy strategic plans. Data were compiled by Paulo Petry of The Nature Conservancy. In total 2,215 projects make up this planned hydropower expansion, which entail adding dams to 673 free-running rivers that are currently free of dams and adding dams to 388 rivers already dammed.