![]() The greenback cutthroat trout (GBCT) were abundant in the late 19th century (Young, Harig, Rosenlund, & Kennedy, 2002), but their populations declined rapidly during the last century due to mining pollution, agriculture, harvesting for commercial sale, and non-native trout invasions (Young et al., 2002 Young & Harig, 2001). One of the native trout in the Southern Rocky Mountains is the Greenback cutthroat trout ( Oncorhynchus clarkii stomias), which is the only salmonid native to the mountain and foothill waters of the South Platte River basin in Colorado (Metcalf et al., 2012). Although increases in summer temperature might benefit age-0 cutthroat trout in the highest-elevation Colorado streams due to a longer growth period (Coleman & Fausch, 2007), diminishing streamflow in summer could amplify the already stressful environment for native cutthroat trout (Mantua, Tohver, & Hamlet, 2010 Roberts et al., 2017). The native cutthroat trout species in the Southern Rocky Mountains have been declining with habitat loss from land-use changes, non-native trout species invasion, and water abstraction from human activities over the last 150 years (Roberts, Fausch, Hooten, & Peterson, 2017). As evapotranspiration increases and precipitation regimes shift because of warming climate, annual mean discharge will likely decrease (Berghuijs, Woods, & Hrachowitz, 2014 Furey, Kampf, Lanini, & Dozier, 2012 Hammond & Kampf, 2020 Jefferson, 2011 Milly & Dunne, 2020 White, Morrison, & Wohl, 2022), resulting in lower base flows in late summer months and a reduction of stream habitat to an extent that could significantly affect coldwater fish (Bradford & Heinonen, 2008 Watts, Grant, & Safeeq, 2016). ![]() Furthermore, warmer temperatures in winter and spring are shifting precipitation patterns from snow-dominant to rain-dominant hydrologic regimes in mountainous regions (Klos, Link, & Abatzoglou, 2014). Climate change in mountainous regions is of particular concern because seasonal snowpack, an important component of regional water supplies, is declining in the western United States (Pederson, Betancourt, & McCabe, 2013 Scalzitti, Strong, & Kochanski, 2016). The size and watershed position of headwater streams makes them especially sensitive to alterations in hydroclimatic conditions caused by changing climate (Beniston, 2003). They also serve as critical habitat for threatened endemic fish species, as well as food sources for fish and other aquatic and riparian organisms (Colvin et al., 2019 Meyer et al., 2007 Schlosser, 1995 Wipfli & Baxter, 2010). In mountainous regions, such as the Rocky Mountains, headwater streams are typically characterized by relatively high gradients with predominately gravel and cobble substrate (Jarrett, 1992 E. Headwater streams, defined as first- and second-order channels, account for nearly 80% of total river length in the United States (US) (E. Tailoring reintroduction and management efforts for GBCT to the individual headwater stream with adequate on-site monitoring could provide a more holistic conservation approach. Future climate change studies related to coldwater fish that examine spatial variation in flow alteration could provide novel data to complement the existing literature on the thermal characteristics. Specifically, streams with step-pool morphologies may experience grater changes in available habitat compared to pool-riffle streams. ![]() Moreover, sites located at a relative higher elevation with larger substrate and steeper slope were projected to experience more reductions in physical habitat due to streamflow reductions. The projected mean summer streamflow shows greater decline (−52% on average) compared to the projected decline in mean August flow (−21% on average). Results illustrate reductions in both predicted streamflow and physical habitat for all future climate projections across study sites. ![]() Thus, this study used field data collected from selected headwater streams across the current distribution of GBCT to construct one-dimensional hydraulic models to evaluate streamflow and physical habitat under four future climate projections. Though previous studies have provided insights on how changes in climate and alterations in stream discharge may affect habitat availability for various native cutthroat trout species, suitable physical habitats have not been evaluated under future climate projections for the threatened Greenback Cutthroat Trout (GBCT) native to headwater regions of Colorado, USA. Coldwater fish are especially sensitive to the altered streamflow and water temperature regimes during summer low flow periods. Headwater streams support vital aquatic habitat yet are vulnerable to changing climate due to their high elevation and small size.
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