Summer temperature in small streams in the Midwestern United States depends on the amount of groundwater inflow, the degree of shading by vegetation, and channel morphology. Water temperature is critical to the coldwater fisheries these streams support, and the controls over spatial variations in their temperature are poorly understood. This research combined field observations and the development of analytical and numerical stream temperature models to evaluate the controls over the temperature of four streams in the Driftless Area of southwestern Wisconsin with drainage areas less than 65 square kilometers. In each watershed, I monitored water temperature and stream base flow in several locations and described riparian vegetation qualitatively. Meteorological conditions were evaluated by monitoring air temperature and short-wave radiation in one location per watershed and obtaining other weather parameters from nearby National Weather Service stations. I developed two one-dimensional models to simulate stream temperature variations along the study reaches. An analytical model solves the steady-state heat transport equation using the equilibrium temperature approximation of the meteorological heat flux. This model illustrates relationships between groundwater discharge, stream width and shade, and it estimates daily mean temperature throughout a reach. I developed a time-dependent finite difference model to simulate daily temperature fluctuations, calibrating it to field data by adjusting shade values for each vegetation type. The models indicate that groundwater provides the greatest cooling in summer where groundwater discharge is large relative to surface-water discharge. Stream reaches where groundwater discharge is concentrated at springs provide the most effective thermal refuges in hot weather. Predicted temperature is affected substantially by variations in stream discharge typical of the historical record. Water temperature is more sensitive to decreases in stream discharge than to increases of the same magnitude, demonstrating the importance of maintaining groundwater recharge. Shade can enhance the impact of groundwater discharge, but this effect may be counteracted by the increased width typical of forested stream reaches. Additional research on the shade and channel morphology under different vegetation types would improve the predictive capabilities of these models.