Rivers in cities mitigate thermal environments on hot summer days. This paper presents a discussion of physical mechanisms of that mitigation effect using field measurements taken in downtown Tokyo.
 Because of heat transfer upward over river surfaces affected by sea breezes, air temperatures near rivers are lower than those in surrounding urban areas. Quadrant analysis revealed that this upward transfer results from the downward motion of colder air, not upward motion of heated air. The cold air mass above intrudes into the hot urban surface layer at the river space.
 In daytime, deep rivers have a water surface that is colder than the air. Consequently, heat is transferred to the water surface. However, shallow river water heats air. The temperature difference between the water surface and the air above reverses at nighttime. These features are attributable to the fact that most net radiation at the water surface is partitioned into heat storage. The effective depth of water for thermal mitigation depends on the time of day.