CHANGES IN THE HYDROLOGY AND SEDIMENT TRANSPORT PRODUCED BY LARGE DAMS ON THE LOWER EBRO RIVER AND ITS ESTUARY

The mean annual flow of the lower Ebro river has reduced by 29% during this century (592 to 426 m(3) s(-1)). The main causes are increased w ater use and evaporation from reservoirs in the river basin. The losse s due to irrigation explain 74% of the decrease, whereas losses by eva poration in the reservoirs explain another 22%. Decreased flow in the lower Ebro river caused an increase in the salt wedge in the estuary. During the study period, the permanent low river flows from July 1988 to April 1990 caused the continuous presence of the salt wedge for 18 months. Historical data for sediment transport in the Ebro river are s carce and incomplete. Limited data before the construction of reservoi rs in the Ebro basin allow only an estimate of the order of magnitude of annual suspended sediment transport (3.0 x 10(7) Mt yr(-1)). Before the construction of large reservoirs in the lower Ebro at the end of the 1960s, the sediment transport was estimated to be around 1.0 x 10( 7) Mt yr(-1). This amount was reduced to around 0.3 x 10(6) Mt yr(-1) after construction of the dam. Currently, this amount ranges from 0.1 to 0.2 x 10(6) Mt yr(-1), which represents a reduction of more than 99 % in sediment transport. On a seasonal scale, the effects of the dams have been the standardization of the river flow and the virtual suppre ssion of peaks in sediment transport. In the estuary, the salt wedge d ynamics changed and its presence increased. River regulation and hydro power generation also changed the hydrology of the river on a daily sc ale. The effect of local storms on the river flow and the sediment tra nsport has been suppressed. At present, these changes are related to h ydropower generation.