An appraisal of the contemporary sediment yield in the Ebro Basin

Purpose The main aim of this work is to provide a comprehensive estimation of the sediment yield of the Ebro Basin (NE Iberian Peninsula) in the twentieth century. Materials and methods For this purpose, we have used the reservoir sedimentation records available for a series of large dams distributed in the catchment. Records started in 1916 and ended in 2001. In addition to the sediment yield estimates, bathymetrical records permit an evaluation of spatial variations in the sediment load of the basin and the assessment of the sediment yield of the lower reaches of the river downstream of major dams. Results and discussion Total sedimentation in the reservoirs of the basin is estimated to be in the order of 670 x 10(6) t. Sediment yield is five times higher in the northern catchments (Pyrenean Range, 370 t km(-2) year(-1)) than in the southern ones (Iberian Massif, 78 t km(-2) year(-1)), reflecting the particular hydromorphic conditions of these areas. Specific sediment yield (SSY) decreases with catchment area but in the southern zone is more scale-dependent. The highest SSY occurs at 350 mm year(-1) showing a decline as runoff increases. SSY of the northern zone plots high in comparison to curves found in the literature, and this is interpreted as the consequence of historical human impact on sediment delivery. The sediment load of the Ebro Basin before the widespread construction of dams and the beginning of land abandonment is estimated to be 21 x 10(6) t year(-1). Conclusions The data illustrate the distinct hydrological and geomorphic behaviour of the two hydro-climatic areas analysed in this study. The SSY data show, for instance, the lower capability of the semi-arid catchments to transfer sediment loads. SSY does not show a linear relation with runoff, a fact that corroborates general patterns previously reported in the literature. Finally, the contemporary sediment load represents around the 1% of what was potentially transported during the beginning of the twentieth century, illustrating the sedimentary deficit of the lower section of the Ebro Basin.