Downstream effects of dams in the fluvial dynamics of the Lower Ebro River

Abstract

The channel of the Lower Ebro River downstream Mequinenza and Ribarroja dams has experienced a series of morphological changes during the second half of the 20th century, mainly: a) lateral erosion, b) colonization of formerly active areas by riverine vegetation and, c) reduction of channel width. Changes have occurred after dam commissioning during the seventies. Dams alter flood frequency and magnitude, which causes a reduction of river capacity to transport sediment. Simultaneously, dams trap most sediment carried by the river from upstream, particularly coarse fractions as bedload, thus the river channel becoming the main downstream sediment source. This study describes the alteration of floods by dams and the subsequent adjustment of the river sediment transport and its morphology, through the analysis of hydrological and geomorphological field data, and historical and recent air photos. Reduction of flood magnitude (up to 25%) is especially important for the small floods in the downstream reaches near to dams. Reduction of flow competence has also diminished river capacity to transport bedload, shifting from a mean annual yield of 400,000 tonnes between 1950 and 1975 to less than 100,000 tonnes afterwards. Morphological changes indicate the river response to such alterations.