Patterns and sources of thermal heterogeneity in small mountain streams within a forested setting Robert J. Danehy a, * , Christopher G. Colson b , Kimberly B. Parrett c , Steven D. Duke d a Boise-Cascade Corporation, P.O. Box 50, Boise, ID, USA b Ecoanalysts, Moscow, ID 83843, USA c Independent Consultant, Hillsboro, OR 97123, USA d Weyerhaeuser Company, P.O. Box 9777, Federal Way, WA 98063, USA Received 19 May 2004; received in revised form 13 December 2004; accepted 13 December 2004 Abstract Spatial thermal patterns and the sources of those patterns were examined in four mountain streams in northeast Oregon and Idaho during hot summer days in 2001. Summer baseflow ranged from <0.007 to 0.22 m 3 /s. Maximum and minimum temperatures increased in the downstream direction, with the diurnal range decreasing with increasing stream size. Each stream was thermally diverse spatially, diurnally, and seasonally, with unique landform and geologic influences on the daily temperature maxima. Insolation was the most important predictor for maximum temperature at each stream. Minimum daily temperatures were less variable than maxima and followed a seasonal and elevation gradient, with minimum air temperature a more important predictor than either the insolation from the previous day or channel features. Models for each stream using insolation and physical habitat characteristics explain 32 to 67% of maximum temperature and 17 to 51% of minimum temperature variability. Groundwater inputs moderated thermal conditions, particularly local maxima, but did not subsume the predominant temperature range controls of insolation and air temperature. # 2004 Elsevier B.V. All rights reserved. Keywords: Streams; Maximum and minimum water temperature; Insolation; Air temperature 1. Introduction Small streams are the most common fluvial feature in any landscape and one of the primary factors that influences and determines their biology is the thermal regime (Beschta et al., 1987). Small streams influence downstream condition and processes, thereby having a strong influence on abiotic conditions and biotic features of entire watersheds (Poff and Ward, 1990; Gomi et al., 2002). A thermal pattern of increasing mean water temperature in the downstream direction occurs in almost all river systems. While decreasing elevation leads to warmer water temperatures at large scales (Ward, 1985), over shorter distances of several www.elsevier.com/locate/foreco Forest Ecology and Management 208 (2005) 287–302 * Corresponding author. Present address: Weyerhaeuser Com- pany, P.O. Box 275, Springfield, OR 97477, USA. Tel.: +1 541 741 5219; fax: +1 541 741 5343. E-mail address: bob.danehy@weyerhaeuser.com (R.J. Danehy). 0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.12.006