Dams – structures and functions – on a global scale:
Nearly 15% of the global annual discharge could be stored in the 28,000 reservoirs world-wide. Benefits of dams and reservoirs are hydropower generation and irrigation water supply, flood protection,domestic and industrial water supply, fish farming, recreation and navigation. Hydropower accounts for 19% of the global power supply. In seven countries more than 50 % of the farmland is irrigated with water from dams.
Due to the transformation of freely flowing rivers into lakes large dams and reservoirs cause environmental impacts. Evaporation losses, increased emissions of methane or hydrogen sulphide and increased health risk due to habitat providing for carrier organisms stand for impacts to the reservoir lakes itself. The biggest impacts in upstream and downstream basins are sediment trapping and river fragmentation.
Large reservoirs reduced world-wide sediment load by 20% and affect erosivity in the river channel and estuarine environments.Fragmentation influence riparian habitats and obstruct the migration of aquatic organisms. 52% of the worlds large river basins are fragmented.