Water bodies in urban area have been expected to mitigate the urban heat island as well as green space. In this paper, we showed the results of observations about heat budget at river water and moving observations to clarify horizontal extent of its thermal effects. Diurnal variations of sensible heat flux estimated by several methods corresponded closely with each other, though these values didn't exceed 100W/m2 even in the peak about midday. Estimated heat storage term occupied more than 90% of net radiation during daytime. According to the numerical simulation, observed water temperature variation required equivalent heat conductivity for actual stirred water about forty times as much as that of stagnant water. This heat balance is completely different from that of green space characterized large latent heat flux. On the other hand, horizontal sensible heat flux from adjacent built-up area to the river space also estimated by eddy correlation method at riverside terrace. Turbulent heat flux across the river-edge amounted to 250W/m2 in daytime, which greatly surpassed vertical sensible heat flux at water surface. The horizontal limits of river-effect along the street crossing the river were about 300m from river-edge, and there was no clear relation between this limits and wind speed.
