As a river flows through shallow littoral regions such as wetlands, forebays, and side arms, the temperature of the water is modified through atmospheric heat exchange. This process, which we call thermal mediation, can control the initial fate of river-borne nutrient and contaminant fluxes within a lake or reservoir. This paper presents temperature observations that demonstrate the occurrence of thermal mediation and directly support the theoretical results derived by Andrad¿ttir and Nepf <2000>. The measurements show that the wetland warms the river inflow by approximately 1--3 ¿C during summer and fall nonstorm conditions. Less thermal mediation occurs during storms, both because the residence time is significantly reduced and because the wetland circulation shifts from laterally well mixed (low flows) to short-circuiting (storms). The dead-zone model can simulate both these regimes and the transition between the regimes and is therefore a good choice for wetland modeling. ¿ 2000 American Geophysical Union