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

Abstract

Based on a 137Cs-derived chronology, we reconstruct the depositional history of the Barasona reservoir in the Esera-Isabena Basin, Central Pyrenees (Spain). Most of the sediments were delivered to the Barasona reservoir during flood events. Comparative mineralogical studies of the Esera and Isabena river channels and the Barasona reservoir sediments help to identify sediment sources and areas of high sediment yield risk, and to establish erosion and sediment transport processes in the watershed. Changes in the mineralogical composition of the sediments can be used to discriminate the relative contribution of the Esera and Isabena rivers during flood periods. Three main periods in the siltation history of the Barasona reservoir were distinguished: (1) From dam construction (1932) to the early 1950s, the reservoir was flushed every year using the lower gates, and average sediment accumulation was very low (1.5 cm-yr). (2) During the 1950s and 1960s, the sedimentation rate increased and reached the highest values: 15 cm/yr, 24.6 cm/yr and 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 (no annual flushes) and climate variability (increased flood frequency and river inflow) caused this increase in sediment delivery and accumulation. (3) The third period was initiated by the enlargement of the dam in 1972. The new sedimentary dynamics in the reservoir caused a general decrease in the sedimentation rate (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 accumulation rate was reduced to 3.7 c,/yr. An increase in channel erosion processes during this period contributed to the transport of sediments to the inner areas of the reservoir.