Author: Miren  Atristain,   (University of the Basque Country)

Description:
Dam decommissioning is booming. During the twentieth century, more than 3,800 dams were removed worldwide, and this activity is likely to increase as many dams approach the end of their useful life. The tallest dam so far decommissioned in Europe is Enobieta Reservoir (42 m, Artikutza, Basque Country, Spain). Project DESEMBALSE follows a Before-After / Control-Impact study design to investigate the geomorphological and ecological consequences of this action. When the reservoir was operating, it acted as a phosphorous sink and an ammonium source and caused coarsening of downstream sediments. Hypolimnetic hypoxia caused mobilisation of metals, iron and manganese, which impaired downstream communities and ecosystem functioning. Below the dam, invertebrate density was 90% lower, chlorophyll-a concentration 46% lower and biofilm showed increased phosphorous uptake. The reservoir was emptied slowly to reduce mobilisation of the sediments stored and to prevent impacts downstream. These impacts are disappearing after drawdown. Furthermore, in the former reservoir, the lotic habitats are recovering swiftly, and colonised by invertebrates, whereas the terrestrial vegetation is colonising the emerged sediments. Overall, the decommissioning eliminated the main black point in an otherwise pristine catchment, with almost no negative effects on fish or invertebrate communities. In addition, the legacy effects of the reservoir seem to be disappearing very fast.  

Category: Scientific Program Abstract > Special Session > CS03 Restoration