Downstream relevance of reservoir management
The management of dams serves many purposes and goals all over the globe, and has important consequences for the downstream rivers and lakes. Among the more than 50,000 so-called large dams, the biggest are located in alpine regions. As a result, the water residence time in heavily dammed alpine valleys typically increased from a few days to several weeks, hydrological regimes shifted seasonally and sediment transport often decreased to half of its natural value. The occurrence of high flows responsible for most particle transport is reduced and particles are trapped behind the dams. These changes modify particle concentrations and particle size distributions, thermal regimes and water quality in downstream waters. As a result, downstream rivers and pre-alpine lakes often experience significant alterations in particle, carbon and nutrient cycling. Also described are common mitigation measures that are often applied in newly-planned damming management.