River regulation can cause direct and indirect alterations in the river thermal cycle, which in turn may affect biological processes. In the lower Ebro River, two weirs (Ascó and Xerta) and a system of three dams (Mequinensa, Riba-roja, Flix) can be found. The weir of Ascó is used for the derivation of water to be used in the cooling system of a nuclear plant, which is returned later to the river and causing an increase of 3°C in the river water temperature. Instead, the weir of Xerta is used for the derivation of water for irrigation which most probably results in a diminution of the thermal inertia of the flowing water mass. On the other side, the system of reservoirs of Mequinensa, Riba-roja and Flix have a seasonal effect on water temperature. Water exiting the system of reservoirs is cooler than the river water entering them in spring and summer and is warmer in autumn and winter. Also, water temperature variability is reduced both in the daily and annual timescales. The reservoir of Riba-roja, receiving the contribution of two main affluents (Ebro and Segre), presents a most interesting hydrodynamic behaviour, that neither is typical lacustrine nor is that of a river. It has a variable pattern of circulation: in winter and early spring, the water column is mixed; in late spring and most of summer, the Segre River water flows above the Ebro River water; and in the rest of summer and most of autumn, the circulation pattern is inverse to the previous one, with the Ebro River water flowing above the Segre River water.