The river water maintains a heat exchange with the atmosphere and with the bottom; this process can reach its equilibrium along a stretch of the river with similar geologic and climatic characteristics. Its behavior can be modified by artificial effects, such as the use of the river water for the cooling of thermal power stations or by the presence of reservoirs. In the case of regions with Mediterranean climate, where the studied reservoirs are located, the effects of changes in the natural thermal regime caused by them produce a trend towards a constant seasonal thermal condition, an increase in daily uniformity, an increase in winter temperatures, and a decrease in summer temperatures. Water temperature is an important factor in the river's ecology, because it greatly conditions the life of fluvial communities. Downstream from power stations or dams, the water temperature evolves in such a way that tends to achieve equilibrium with the environment.