A comparison of surface and subsurface controls on summer temperature in a headwater stream Ryan J. MacDonald, 1 * Sarah Boon, 1 James M. Byrne 1 and Uldis Silins 2 1 University of Lethbridge, 4401 University Dr. W., Lethbridge, AB, T1K 3M4, Canada 2 University of Alberta, 809 General Services, Edmonton, AB, T6G 2H1, Canada Abstract: This study compared summer stream temperature between two years in the Star Creek catchment, Alberta, a headwater basin on the eastern slopes of the Canadian Rocky Mountains. Star Creek is a subsurface water dominated stream, which represents important habitat for native salmonid species. Hydrometeorological data from May to September of 2010 and 2011 accompanied by stream energy budget calculations were used to describe the drivers of stream temperature in this small forested stream. Mean, maximum, and minimum weekly stream temperatures were lower from May to August and higher in September 2011 compared to 2010. Weekly range in stream temperature was also different between years with a higher range in 2010. Inter-annual stream temperature variation was attributed discharge differences between years, shown to be primarily governed by catchment-scale moisture conditions. This study demonstrates that both meteorological and hydrological processes must be considered in order to understand stream temperature response to changing environmental conditions in mountainous regions. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS stream temperature; hydrology; subsurface Received 22 March 2012; Accepted 1 February 2013 INTRODUCTION Stream temperature controls aquatic ecosystem function by directly influencing water quality, ecosystem productivity, and the physiological functioning of aquatic organisms (Allan, 1995; Caissie, 2006; Webb et al., 2008). For native salmonids in particular, maintaining cold-water habitat during the summer is critical for survival (Rieman et al., 2007; Isaak et al., 2010). Natural and anthropogenic disturbances such as drought, wildfire, insect infestation, and industrial development affect catchments across North America. These disturbances can significantly alter stream temperature regimes (Poole and Berman, 2001; Morrison et al., 2002; Mohseni et al., 2003; Dunham et al., 2007), thus process studies that address controls on stream temperature have become increasingly important. Stream temperature is governed by changes in the energy budget of the stream, with solar radiation constituting the dominant non-advective source of heat input during the summer. Solar radiation is supplemented by latent and sensible heat exchanges at the air–water interface, which have a smaller contribution to the stream energy budget in small forested streams (Brown 1969; Brown and Krygier, 1970; Sinokrot and Stefan, 1993; Webb and Zhang 1997; Johnson and Jones 2000; Hannah et al., 2008; Leach and Moore, 2010). In addition to atmospheric controls, factors such as riparian shading, local topography, streambed conduction, friction, substrate type, and surface–subsurface interactions can also contribute to stream temperature regimes (Theurer et al., 1984; Webb and Zhang, 1997; Story et al., 2003; Hannah et al., 2004; Johnson, 2004; Moore et al., 2005; Brown et al., 2006; Hannah et al., 2008; Leach and Moore, 2010; Leach and Moore, 2011; Guenther et al., 2012). Considering the range of processes controlling stream temperature, an understanding of these processes and their relative contributions in systems dominated by subsurface contribution to streamflow is important for many fresh water ecosystems. Subsurface-dominated streams, typically constrained to headwater mountain regions, are critical to the survival of organisms such as salmonids, as many populations at-risk are currently limited to headwater streams where historically they inhabited much larger ranges (Behnke, 2002). Water originating from the subsurface typically cools streams during summer and warms them in winter which provides thermal refugia, habitat for rearing, governs the selection of spawning locations, and affects stream water quality (Ward, 1994; Power et al., 1999). Changes in thermal regime resulting in habitat loss and increased hybridization with non-native species, therefore, pose a substantial threat to native salmonids in headwater systems (Muhlfeld et al., 2009; Isaak et al., 2011). This study is focused in the Star Creek catchment, southwestern Alberta, which is representative of habitat of two cold-water adapted species: native westslope cutthroat trout (Oncorhynchus clarki lewisii ) and bull trout (Salvelinus confluentus), listed as threatened and species of special concern, respectively (ASRD, 2009; COSEWIC, 2009). To-date, there have been no process-based stream *Correspondence to: Ryan J. MacDonald, University of Lethbridge, 4401 University Dr. W., Lethbridge, AB T1K 3M4, Canada. E-mail: ryan.macdonald@uleth.ca HYDROLOGICAL PROCESSES Hydrol. Process. 28, 2338–2347 (2014) Published online 8 April 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9756 Copyright © 2013 John Wiley & Sons, Ltd.