We remain desperately dependent on water for our survival. Freshwater is so valuable that even today with our sophisticated hydroengineering systems, more than half the world's population lives within 3 kilometres (2 miles) of a surface body of freshwater such as a lake or river. Less than 10% live more than 10 km (6 miles) away.
With seven billion of us now needing water, agriculture, industry and to generate power, the human demand on global water supplies has never been greater. And much of this is at the expense of the other water users on this planet – all species need water, and ecosystems around the world are suffering from the decline in supply.
Our problem is that 97% of our planet's water is salty ocean. Of the 2.5% that is fresh, most of it is trapped in glaciers in Antarctica, Greenland and mountains. Just 0.008 of 1% of all the water on Earth is held on the surface in rivers, lakes and wetlands where we can get at it. An even tinier proportion hangs in the atmosphere as clouds and rain.
Since ancient times people have found many ways to store water – from trapping roof run-off to underground monsoon storage. But over the past two centuries, humans have carried out hydroengineering on an increasingly grander scale. Dams are a great way to store water for release during drought, to modulate damaging floods, and to provide electricity by passing falling water through turbines.
Two-thirds of the world's major rivers have now been disrupted with more than 50,000 dams in an attempt to store water and provide power. In the US, there are more than 85,000 dams, stoppering large and small rivers, and in most cases utterly transforming natural flow. The most famous of these, the Hoover Dam, constructed in the 1930s, is largely responsible for the fact that the mighty Colorado River no longer reaches the ocean.
And that's the problem. Dams, for all their attractive benefits, are saddled with an overflow of negative impacts. Creating a reservoir means a large area must be flooded – often prime riverside land. Communities may lose their land, houses and culturally important sites such as ancestral burial grounds, or a landscape that carries strong meaning for them.
Environmentally, the new reservoir can be a haven for wildlife, especially birds; however, inappropriate flooding of vegetation can cause greenhouse gas emissions and poison the water for fish. Also, the dramatic rise and fall of water levels during dam releases – sometimes of several metres – is too extreme for plants and animals to cope with, resulting in dead zones around the shores of reservoirs. Fish that lay their eggs in the shallows among submerged tree roots, for example, may find a few hours later that those sites are high and dry with the eggs desiccated.
Downstream of a dam, the seasonal floods that revitalize wetlands and fertilise paddy fields cease. The flow may be so reduced that farmers cannot irrigate their fields and streams are no longer navigable. In many cases the upstream-downstream demands straddle national borders leading to conflict over precious water, disputes have been ongoing between India and Pakistan, and Turkey, Syria and Iraq, for example.
Dams don't just block water flow. They also prevent fish migrations up- and downstream, and dams are a barrier to sediment flows. Instead of being flushed downriver, sediments get backed up against the dam walls, which damages turbines and causes the reservoir level to increase over time. Downstream, though, the effects of losing nutrient-rich sediments is far more problematic. The fertility of the entire system can be affected, with soils lost during seasonal rains not being replaced.