RIVER RESEARCH AND APPLICATIONS River Res. Applic. 21: 849–864 (2005) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rra.855 FLOW REGIME ALTERATIONS UNDER CHANGING CLIMATEIN TWO RIVER BASINS: IMPLICATIONS FOR FRESHWATER ECOSYSTEMS C. A. GIBSON, a * J. L. MEYER, a N. L. POFF, b L. E. HAY c and A. GEORGAKAKOS d a Institute of Ecology, University of Georgia, Athens, GA 30602, USA b Department of Biology, Colorado State University, Fort Collins, CO 80523, USA c US Geological Survey, Box 25046, MS 412, Denver Federal Center, Denver, CO 80225,USA d School for Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA ABSTRACT We examined impacts of future climate scenarios on flow regimes and how predicted changes might affect river ecosystems. We examined two case studies: Cle Elum River, Washington, and Chattahoochee–Apalachicola River Basin, Georgia and Florida. These rivers had available downscaled global circulation model (GCM) data and allowed us to analyse the effects of future climate scenarios on rivers with (1) different hydrographs, (2) high future water demands, and (3) a river–floodplain system. We compared observed flow regimes to those predicted under future climate scenarios to describe the extent and type of changes predicted to occur. Daily stream flow under future climate scenarios was created by either statistically downscaling GCMs (Cle Elum) or creating a regression model between climatological parameters predicted from GCMs and stream flow (Chattahoochee–Apalachicola). Flow regimes were examined for changes from current conditions with respect to ecologically relevant features including the magnitude and timing of minimum and maximum flows. The Cle Elum’s hydrograph under future climate scenarios showed a dramatic shift in the timing of peak flows and lower low flow of a longer duration. These changes could mean higher summer water temperatures, lower summer dissolved oxygen, and reduced survival of larval fishes. The Chattahoochee–Apalachicola basin is heavily impacted by dams and water withdrawals for human consumption; therefore, we made comparisons between pre-large dam conditions, current conditions, current conditions with future demand, and future climate scenarios with future demand to separate climate change effects and other anthropogenic impacts. Dam construction, future climate, and future demand decreased the flow variability of the river. In addition, minimum flows were lower under future climate scenarios. These changes could decrease the connectivity of the channel and the floodplain, decrease habitat availability, and potentially lower the ability of the river to assimilate wastewater treatment plant effluent. Our study illustrates the types of changes that river ecosystems might experience under future climates. Copyright # 2005 John Wiley & Sons, Ltd. key words: climate change; freshwater ecosystems; flow regimes; river; ecological integrity; Indicators of Hydrologic Alteration INTRODUCTION The quantity and timing of river flow are critical components to water supply, water quality, and the ecological integrity of river systems (Poff et al., 1997). The five components of flow regime (magnitude, frequency, duration, timing and rate of change) influence the ecological dynamics of river systems directly and indirectly through their effects on other primary regulators (Karr, 1991; Poff et al., 1997). Alteration of any of these flow parameters can have dramatic effects on aquatic organisms, riparian species, energy flow in the system, sediment movement and floodplain interactions (Poff et al., 1997, and references therein). Hence, climatic changes that create shifts in the timing and magnitude of low or high flow events or change the magnitude of river flow at monthly, seasonal, or yearly time scales could result in dramatically altered river systems. There have been numerous studies that have examined the potential impacts of future climate scenarios on aquatic ecosystems at a regional scale (e.g. Hauer et al., 1997; Melack et al., 1997; Moore et al., 1997; Mulholland et al., 1997; Meyer et al., 1999; Stone et al., 2001). These studies give insight to the type of changes that might be expected for aquatic ecosystems and their inhabitants under future climate scenarios, including changes in species Received 5 March 2003 Revised 15 September 2004 Copyright # 2005 John Wiley & Sons, Ltd. Accepted 30 October 2004 * Correspondence to: C. A. Gibson, Darrin Freshwater Institute, 5060 Lake Shore Drive, Bolton Landing, NY 12814, USA. E-mail: gibson.cathy@gmail.com