Sediment sources and siltation in mountain reservoirs: a case study from the Central Spanish Pyrenees

Based on a Cs-137-derived chronology, we reconstruct the depositional histo ry of the Barasona reservoir in the Esera-Isabena Basin, Central Pyrenees ( Spain). Most of the sediments were delivered to the Barasona reservoir duri ng flood events. Comparative mineralogical studies of the Esera and Isabena river channels and the Barasona reservoir sediments help to identify sedim ent sources and areas of high sediment yield risk, and to establish erosion and sediment transport processes in the watershed. Changes in the mineralo gical composition of the sediments can be used to discriminate the relative contribution of the Esera and Isabena rivers during flood periods. Three m ain periods in the siltation history of the Barasona reservoir were disting uished: (1) From dam construction (1932) to the early 1950s, the reservoir was flushed every year using the lower gates, and average sediment accumula tion was very low (1.5 cm/yr). (2) During the 1950s and 1960s, the sediment ation rate increased and reached the highest values: 15 cm/yr, 24.6 cm/yr a nd 18.2 cm/yr in the Esera river mouth, the northern and the southern areas of the reservoir, respectively. Both changes in the exploitation regime (n o annual flushes) and climate variability (increased flood frequency and ri ver inflow) caused this increase in sediment delivery and accumulation (3) The third period was initiated by the enlargement of the dam in 1972, The n ew sedimentary dynamics in the reservoir caused a general decrease in the s edimentation rare (11.6 cm/yr in the southern areas; 10 cm/yr in the Esera mouth) that was especially noticeable in the northern areas, where the accu mulation rate was reduced to 3.7 cm/yr. An increase in channel erosion proc esses during this period contributed to the transport of sediments to the i nner areas of the reservoir. (C) 1999 Elsevier Science B.V. All rights rese rved.