The greenish line in the following chart shows trends in the number of global free-flowing rivers greater than 1,000 kilometers in length. The number has declined from 160 in 1900 to approximately 60 today and is projected to fall further to 2020.
The bars in the chart show the number of big rivers damned in each decade of the last century. Notable is the big surge in the decades after World War II and the revival projected over the next decade.
The chart comes from a 2006 report by the World Wildlife Fund, from which Desdemona Despair provides a few excerpts:
The rapid development of water management infrastructure – such as dams, dykes, levees, and diversion channels – has left very few rivers entirely free flowing. Of the approximately 177 rivers greater than 1,000 kilometers in length, only around a third remain free-flowing and without dams on their main channel.
While clearly this infrastructure provides benefits at one level, such as hydropower or irrigation, there is often a hidden cost to aquatic ecosystems and the wider ecosystem services that they provide. In order to sustain the wealth of natural processes provided by freshwater ecosystems – such as sediment transport and nutrient delivery, which are vital to farmers in floodplains and deltas; migratory connectivity, vital to inland fisheries; and flood storage, vital to downstream cities – it is imperative to appreciate the importance of free flowing rivers, and developing infrastructure with a basin-wide vision.