DAMMED IF YOU DO, DAMNED IF YOU DON'T

Naturally flowing rivers are among the most dynamic ecosystems on Earth, with enormous spatial and temporal complexity. Streamflow defines the physical template of riverine ecosystems (Poff et al., 1997), provides longitudinal and lateral access to foraging, spawning and recruitment habitat (Junk et al., 1989), and acts as an evolutionary selective force and an ecological filter of various survival strategies employed by aquatic and riparian organisms (Townsend & Hildrew, 1994; Jackson et al., 2001; Lytle & Poff, 2004). At the same time, human society requires water for life. Over the millennia, humans have altered streamflow in riverine systems for myriad reasons including harnessing water for drinking, irrigation and recreation, and providing flood control and hydropower (Gleick, 2003). The freshwater footprint of humanity stamps the entire globe, with nearly half of major river systems affected by dams (Vörösmarty et al., 2010; Lehner et al., 2011). The future construction of dams, particularly in economically developing nations, is an inevitable consequence of human population growth and increasing freshwater and electricity needs in a changing climate (Palmer et al., 2008; McDonald et al., 2012).

Despite providing many societal benefits, river regulation by dams has also caused considerable ecological damage and the loss of important ecosystem services valued by society. Dams fragment rivers, creating strings of artificial lakes punctuated by barriers that are often impassable by fish, and they alter physical riverine habitat and water quality in both upstream and downstream directions (Nilsson et al., 2005; Reidy Liermann et al., 2012). In particular, dams are the primary driver of hydrologic change throughout the United States (Carlisle et al., 2011), resulting in reduced flow seasonality and variability and generally increasing short-term minimum flows while decreasing short-term maximum peaks (Poff et al., 2007). These changes alter the historical disturbance regime, rendering some biotic adaptations to these regimes obsolete while potentially favouring others. For example, reduced flow variability by dams has been associated with significant losses of native fish species (Meador & Carlisle, 2012) while concurrently creating new niche opportunities above and below dams that are often occupied by non-native fishes (e.g. Olden et al., 2006; Johnson et al., 2008).