Morpho-sedimentary evolution of the middle-course reaches of the Segre River

In this study, the hydrological and morpho-sedimentary status of the medium reaches of the River Segre is examined from the analysis of the alteration of the hydrological and floods regime and of the sedimentary imbalance in the second half of the 20th century. In this period, the river has been significantly altered due to the construction of dams and to the in-channel gravel mining. Results show that the alteration of the flow regime downstream from the Oliana Dam is moderate, with the more evident change affecting the magnitude and frequency of floods. The construction of Rialb Dam has magnified channel changes. Sediment retention in reservoirs and the massive in-channel gravel mining have caused a remarkable sedimentary imbalance. The estimated sediment retention is between 87 and 96% (100% for bedload). This impact, along with an intense in-channel mining that clearly exceeded the mean sedimentary load of the river, have considerably increased the current sedimentary deficit and reinforced the hydrodynamic imbalance (evident from the observations of armouring and incision in the river bed). The morphological evolution of the river, analysed by means of the Index of Fluvial Dynamism (IFD), shows a tendency towards stabilisation due to the encroachment of riparian vegetation after the closing of the Oliana Dam in 1959. This tendency was temporarily altered by the great flood of 1982. The apparently new fluvial dynamism after this event gradually decreased afterwards. Nowadays, riverbed mobility has definitely decreased and the riparian vegetation encroachment makes the low flow channel to occupy practically all the active river bed. The main morphosedimentary units (lateral and central bars) have disappeared. Results can be used to design hydrosedimentary restoration plans, which include maintenance of artificial floods and gravel injection. Maintenance floods and bypass sediments through reservoirs may reactivate the riverbed, thus reducing the energy and mass imbalances, whereas gravel injections would reduce incision and would re-create habitats and morphological units.