How Dams Vary and Why It Matters for the Emerging Science of Dam Removal

D are structures designed by humans to capture water and modify the magnitude and timing of its movement downstream. The damming of streams and rivers has been integral to human population growth and technological innovation. Among other things, dams have reduced flood hazard and allowed humans to settle and farm productive alluvial soils on river floodplains; they have harnessed the power of moving water for commerce and industry; and they have created reservoirs to augment the supply of water during periods of drought. In the 5000 or so years that humans have been building dams, millions have been constructed globally, especially in the last 100 years (Smith 1971, WCD 2000). If dams have successfully met so many human needs, why is there a growing call for their removal? The answers to this question require an appreciation of society’s changing needs for, and concerns about, dams, including the emerging recognition that dams can impair river ecosystems (Babbit 2002). But decisions about dam removal are complex, in no small part because great scientific uncertainty exists over the potential environmental benefits of dam removal. Certainly, the scarcity of empirical knowledge on environmental responses to dam removal contributes to this uncertainty (Hart et al. 2002). More fundamentally, however, a scientific framework is lacking for considering how the tremendous variation in dam and river attributes determines the ecological impacts of dams and the restoration potential following removal. Such an ecological classification of dams is ultimately needed to support the emerging science of dam removal. In this article, we develop a conceptual foundation for the emerging science of dam removal by (a) reviewing the ways that dams impair river ecosystems, (b) examining criteria used to classify dams and describing how these criteria are of limited value in evaluating the environmental effects of dams, (c) quantifying patterns of variation in some environmentally relevant dam characteristics using governmental databases, (d) specifying a framework that can guide the development of an ecological classification of dams, and (e) evaluating the ways that dam characteristics affect removal decisions and the future of dam removals. We restrict our analysis to the United States, where dam removals are currently hotly debated; however, the ecological framework we advocate could also be generalized to other parts of the world.