Managing for Variability to Sustain Freshwater Ecosystems

Water resources development for human needs has come at a significant cost to the natural functioning of aquatic and riparian ecosystems in the United States and around the world. The construction of hundreds of thousands of dams nationally and globally and the extensive human transformation of land cover have greatly modified the magnitude, timing, and duration of river flows that have historically sustained naturally dynamic riverine ecosystems Graf 1999; Nilsson et al. 2005; Poff et al. 2006 . This pervasive alteration is contributing to the global crisis of biodiversity loss in freshwater ecosystems and to the degradation of many of the natural goods and services that these ecosystems provide to human communities Postel and Richter 2003; Dudgeon et al. 2006; Poff et al. 2007 . The importance of sustaining healthy ecosystems to support human well-being in a world of shrinking water resource availability is now broadly recognized World Commission on Dams 2000; Gleick 2003; Millennium Ecosystem Assessment 2003 . The many economic benefits provided to human society from maintaining healthy aquatic and riparian ecosystems, coupled with the high cost and difficulty of restoring degraded ecosystems Bernhardt et al. 2005 , have fueled the growing awareness that principles of ecosystem science need to be more fully integrated into water resources planning and management. The term “environmental flows” has emerged to capture the important idea that nature must be allocated a share of water in water resources management if such planning and development is truly to be integrated integrated water resources management or IWRM Naiman et al. 2002; Bernhardt et al. 2006 . Environmental flows are defined by the Brisbane Declaration http://www.river symposium.com/index.php? element 2007 BrisbaneDeclaration 241007 as the quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems. In recent years, guidelines for environmental flows have been developed and variously implemented in many places in the United States and around the world. Such developments are certainly encouraging; however, a key issue in actually achieving ecologically sustainable development revolves around just how these environmental flows are defined and implemented. All too often, there is a precarious balance between provision of even minimal environmental flows and real or perceived human demand, leading to social conflict and associated legal or regulatory challenges Poff et al. 2003 . Maintaining adequate environmental flows in heavily allocated basins is especially difficult and is becoming more commonplace—the Klamath River basin Oregon and California , the Appalachicola-ChatahoocheeFlint basin Georgia, Alabama, and Florida , and the recent drought conditions in the city of Atlanta are but a few contemporary examples of conflicts that threaten to undermine the political will necessary to sustain healthy riverine ecosystems. Such con-