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Half of the world鈥檚 river systems host hydro-electric dams. They offer reliable electricity but their construction forces people from their homes and disrupts the natural life of the river.

Scores of dams already span the Mekong River, the great waterway linking China to Vietnam. They鈥檝e brought power and jobs to some of the most undeveloped parts of South-East Asia and the building boom shows no sign of ending. But the impact of the massive building programme on those living in the Mekong Delta and along the river is immense: silt deposits are disrupted and fish populations are displaced, as are many of the millions of people that depend on them.

Reporter Peter Hadfield sails up the Mekong to meet those communities living with the dams on their doorstep and discover how their lives are impacted.

Meanwhile, presenter Didi Akinyelure is in western Europe to find out why the countries that pioneered hydro-power are now turning their backs on it. In Switzerland they are releasing floodwater from their dams to bring life back to a tamed mountain wilderness. In France dams are actually being dismantled to revive fish life on Normandy鈥檚 rivers.

So how should we feel about dams? Do developing countries need the reliable low-carbon electricity they provide? Can they be built in less damaging ways or should we call a halt to the age of the mega-dam?

(Photo: Ota Khami, 55, stands where his home use to be before it was bulldozed to make way for the Sesan 2 dam in Stung Treng, Cambodia. Credit: Getty Images)