PerspectiveINFRASTRUCTURE

Can dams be designed for sustainability?

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Science  08 Dec 2017:
Vol. 358, Issue 6368, pp. 1252-1253
DOI: 10.1126/science.aaq1422

Village fishermen exploit the predictable bounty of the annual flood in the Lower Mekong Basin.

PHOTO: AP/DAVID GUTTENFELDER/NATIONAL GEOGRAPHIC CREATIVE

Water, food, and energy security are cornerstones of a sustainable and prosperous future. Rivers play a key role in delivering critical ecosystem services that contribute to this security on a global scale. But decisions concerning how rivers are harnessed or otherwise used for the benefit of human societies inevitably create environmental and social conflicts, particularly when new dams threaten valued ecosystem services. Given the proposed proliferation of thousands of new dams in many developing countries (1), can policy-makers and managers be smarter in dam design and operation to reduce these inherent conflicts to generate valuable co-benefits? On page 1270 of this issue, Sabo et al. (2) propose a novel approach to designing flow releases from existing and proposed dams in the Mekong River Basin in order to sustain a highly valued and threatened natural fishery.

In Southeast Asia, the Mekong River and its tributaries support a natural floodplain fishery that ∼60 million people rely on directly for their sustenance and livelihoods. Six upstream dams currently exist on the Mekong River, and another 13 are planned to provide hydroelectric energy and stored irrigation water for the people of China, Cambodia, and Laos. Operation of existing dams and construction of new dams pose a great risk to the region's sustainable fisheries, both by erecting migration barriers for spawning fish and by altering the seasonal cycle of flooding and draining of the vast lowland areas required to support fish production (3). Can dams, which inevitably alter natural river hydrology, be operated or designed to meet the fishery's hydrological needs?

To answer this question, Sabo et al. analyzed more than a decade of annual fisheries catch in the Tonle Sap River-floodplain complex of Cambodia, a key source of fish production in the lower Mekong. Using spectral analysis techniques, they identified dominant signals in the hydrology time series that best explain interannual variation in fish harvest. On the basis of the resulting fish-hydrology relationships, they propose an engineered flow regime for the Tonle Sap that reflects elements of the historical, pre-dam flow regime in the basin, but with some key differences. Seasonal floodplain drying would be prolonged, followed by a very rapid transition to seasonal flooding. The authors find that the statistically optimized designer flow regime would lead to a two- to fourfold increase in fisheries catch compared with the historical (pre-dam) natural flow regime. The water volumes needed to create the seasonally timed designer flow already exist in upstream water storage dams. New dams could be operated to contribute to the designer flow.

Considerable political motivation would be required to coordinate the multinational interests in the Mekong Basin to manage toward such a designed flow regime. Nonetheless, Sabo et al.'s study illustrates how dams can be operated to improve an essential fishery for millions of people while also providing energy and water security needs. Sabo et al.'s model approach is, in principle, transferable to other river basins where similar threats to sustainable fisheries are posed, such as the Amazon River Basin, which is also slated for extensive hydropower dam expansion (4, 5).

As conflict over water resources increases under growing population demands and accelerating climatic change, designing flows in dam-regulated rivers to meet both human consumption needs and valued ecosystem functions will become more pressing (6). The world's rivers are extensively modified by tens of thousands of dams (7), resulting in many lost ecosystem services, including native fisheries (8). Strategic flow releases from dams to maintain or restore degraded ecosystem functions and ecological targets are now receiving greater attention from river scientists and managers (9, 10), and these can potentially be balanced with human demands. For example, in drought-prone rivers, flows can be designed to favor native over non-native fish species while simultaneously meeting human water needs (11).

The Sabo et al. paper reinforces the growing understanding that water managers can, and must, balance socioeconomic and ecological needs for regulated rivers. Incorporating flexibility into the design of new dams will enable proactive maintenance of downstream ecosystem services, even as future hydrology and social demands for water change (6). Such flexibility will be critical for adaptive management; it should be mandated for all new dam constructions and selectively required for existing dams.

With the likely expansion of the designer flow philosophy, careful evaluation of limitations and uncertainties is needed. For example, Sabo et al.'s work shows that production of key species in the Tonle Sap fishery is likely to be enhanced under a designed flow regime; however, it remains unclear whether such engineered flows support broader biodiversity that may promote sustainable fish populations.

Ultimately, managing rivers for multiple, sustainable benefits requires integrating scientific, social, and policy perspectives into operational decision frameworks (12). In rivers, optimizing among divergent stakeholder interests remains a challenge because flow-dependent benefits are distributed in complicated and often unspecified ways (13). For example, the water-dependent cultural values of local or indigenous peoples have historically been largely excluded from water management decisions and are therefore often deemphasized in decisions about building and operating water infrastructure (14). In the Mekong River, cultural values and economic livelihoods align around the native fisheries. Given this, Sabo et al.'s work is an exemplar of how ecological science can inform equitable distribution of river-dependent benefits toward achieving social-ecological sustainability in complex and contested river ecosystems.

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