Introduction to the special issue: understanding and linking the biophysical, socioeconomic and geopolitical effects of dams.

Dams have made important contributions to human development, and the benefits derived from them have been considerable (World Commission on Dams, 2000). With the rising global population and desire to increase quality of life, dams are prominently staged to deliver hydropower, irrigation and drinking water supplies, recreation, navigation, and many other resources to the growing planet. Further, the uncertainty of the future climate regime may mean that dams will play an increasingly important role in water resources. For example, it is predicted that droughtaffected areas will likely increase in extent and flood risks will be augmented in response to increased frequency of heavy precipitation events (IPCC, 2007), the effects of both of which may be ameliorated by dams. Thus, the construction of new dams, while in decline in the United States since the 1970s (US Army Corps of Engineers, 1996), may see a renewed intensity both at home and abroad. At the same time, dams are increasingly slated for removal, reflecting a growing concern over their adverse ecological, social, and economic impacts (Pejchar and Warner, 2001). Aging structures, which can pose a risk to public safety, are increasingly removed under new policies and funding sources to support removal projects (Heinz Center, 2002). Dam removal is also emerging as a promising option for restoring continuums and reconnecting habitats for migratory fish species, including anadromous salmon, that are federally listed as threatened or endangered in the United States (Gregory et al., 2002). However, there is a great deal of uncertainty about the consequences of dam removal (Aspen Institute, 2002; Hart et al., 2002), particularly the unknowns related to the extent, magnitude, and timing of physical and ecological outcomes (Heinz Center, 2002; Hart et al., 2002). Thus, dams are a crucial issue for resource managers, scientists and policymakers. To advance the knowledge about and inform the management of dams, this special issue of the Journal of Environmental Management presents cutting-edge research in various academic disciplines and proposes new multi-disciplinary approaches for understanding and predicting how dams and dam removals affect societies and ecosystems. The goal of this collection is to offer guidance for and provoke conversations about the interdisciplinary nature of dams. The idea for the special issue was conceived at a symposium, held in April of 2007 at Skamania Lodge on the Columbia River in Washington State, USA, which brought together scientists and resource managers to discuss the effects of dams on social and ecological systems. Many of