Development and decommissioning of small dams : analysis of impact and context

Abstract

Management of small dams may have profound implications for the health and integrity of small rivers and freshwater diversity. Global indicators suggest future growth in the small hydropower sector, particularly in developing countries. As a renewable energy source, it is often presumed that small hydropower entails fewer and less severe negative externalities than those associated with development of larger hydropower facilities. However, as very little study exists investigating the cumulative effect of small‐scale hydrodevelopment on ecological and social systems, limited data is available to substantiate this assumption. To enhance understanding of small hydropower effects, I present a comprehensive comparison of biophysical consequences of small and large hydropower stations in Nujiang Prefecture, Yunnan Province, China. My comparison indicates that cumulative ecological effects of small hydropower development may equal or exceed those associated with development of large hydropower. Because negligible adverse effects are anticipated, small hydropower projects often are characterized by reduced project oversight, streamlined permitting processes, relaxed requirements for environmental impact assessment and mitigation, and increased opportunity for international support relative to large hydropower projects. Outcomes from my comparison of small and large hydropower stations in Nujiang Prefecture and synthesis of available study from diversion‐design facilities suggest that current policies allowing less rigorous standards of impact assessment for the small hydropower sector should be reevaluated. Further study is necessary to optimize management that minimizes the consequences of small hydropower and to create effective policies encouraging low‐impact development of renewable energy sources. With regard to decommissioning of small dams, the restoration potential of small rivers impounded by reservoirs filled with coarse sediment may outweigh the likelihood of detrimental dam‐removal effects. After removal of the Brownsville Dam from the Calapooia River, Oregon, aquatic habitats directly below the dam became more heterogeneous over the short term, while changes further downstream were virtually undetectable. As the Brownsville Dam stored coarse rather than fine sediments, outcomes following removal differ from results of many prior dam removal studies. As current conceptual ideas of dam removal effects are largely informed by study of low‐gradient systems transporting finer sediments, I propose a refined conceptual model describing downstream geomorphic processes following small dam removal when upstream fill is dominated by coarse sediments. Dam removal monitoring studies challenge many basic assumptions of conventional experimental designs and data analysis techniques, thus the quality of information available to aid decision making may often be questionable or misleading. To assist managers in undertaking dam removal monitoring, I also discuss assets and limitations of monitoring and analysis options available for dam removal studies, with emphasis on selecting a rigorous experimental design and determining significance of results.