ABSTRACT: One of the biggest challenges in managing cold water streams in the Midwest is understanding how stream temperature is controlled by the complex interactions among meteorologic pro- cesses, channel geometry, and ground water inflow. Inflow of cold ground water, shade provided by riparian vegetation, and channel width are the most important factors controlling summer stream temperatures. A simple screening model was used to quantitatively evaluate the importance of these factors and guide management decisions. The model uses an analytical solution to the heat trans- port equation to predict steady-state temperature throughout a stream reach. The model matches field data from four streams in southwestern Wisconsin quite well (typically within 1˚C) and helps explain the observed warming and cooling trends along each stream reach. The distribution of ground water inflow throughout a stream reach has an important influence on stream temperature, and springs are especially effective at providing thermal refuge for fish. Although simple, this model provides insight into the impor- tance of ground water and the impact different management strate- gies, such as planting trees to increase shade, may have on summer stream temperature. (KEY TERMS: stream temperature; modeling; watershed manage- ment; ground water hydrology; meteorology/climatology; surface water hydrology.) Gaffield, Stephen J., Kenneth W. Potter, and Lizhu Wang, 2005. Predicting the Summer Temperature of Small Streams in Southwestern Wisconsin. Journal of the American Water Resources Association (JAWRA) 41(1):25-36. INTRODUCTION Importance One of the most difficult challenges in managing cold water stream fisheries is understanding the controls of summer water temperature. Water temper- ature affects physiological processes and the distribu- tion of aquatic species, and it limits productivity and the rates of important biochemical processes (Dunne and Leopold, 1978; Diana 1995). The distribution of brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) in Midwestern streams is typically lim- ited by stream temperature (Becker, 1983). It is wide- ly recognized that streams in this region are cooled by inflows of cold ground water (Becker, 1983; Bowlby and Roff, 1986; Meisner et al., 1988; Field and Graczyk, 1990; Meisner, 1990; McRae and Edwards, 1994; Poff and Allan, 1995; Seelbach and Wiley, 1997; Seelbach et al., 1997; Younus et al., 2000; Poole and Berman, 2001). However, shade from streamside veg- etation, stream channel geometry, and weather condi- tions also have an important impact on stream temperature (Younus et al., 2000; Poole and Berman, 2001; Blann et al., 2001). A lack of understanding of the details of how these factors control summer water temperature in small Midwestern streams makes habitat management decisions difficult. It is widely accepted that springs and other areas of concentrated ground water discharge create cold refuges for fish during the summer. Field surveys of small streams in the Sierra Nevada Mountains (Con- stantz, 1998) and Wisconsin (McRae and Edwards, 1994) have shown a correspondence between high rates of ground water discharge and cold summer stream temperature. However, little quantitative information is available about how the spatial distri- bution of ground water inflow affects stream tempera- ture. Many previous studies have focused on heat 1 Paper No. 02127 of the Journal of the American Water Resources Association (JAWRA) (Copyright © 2005). Discussions are open until August 1, 2005. 2 Respectively, Senior Hydrologist, Montgomery Associates: Resource Solutions, LLC, 2820 Walton Commons West, Suite 135, Madison, WI 53718; Professor, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706-1691; and Supervisory Research Biologist, Michigan Department of Natural Resources, Institute for Fisheries Research, 1109 North University Drive, Ann Arbor, Michigan 48109 (E-Mail/Gaffield: steve@ma-rs.org). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 25 JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION FEBRUARY AMERICAN WATER RESOURCES ASSOCIATION 2005 PREDICTING THE SUMMER TEMPERATURE OF SMALL STREAMS IN SOUTHWESTERN WISCONSIN 1 Stephen J. Gaffield, Kenneth W. Potter, and Lizhu Wang 2