John Vidal’s first stop along the river is the tiny country of Laos. Fifty years ago, Laos began to build a series of giant dams. It was the first chance the country had to generate the electricity and money needed to emerge from deep poverty. But is this demand for clean energy creating ecological and human havoc?

Khamlouvilaivong Vanthong’s first job as a civil engineer was on the Nam Ngum dam. It was 1968 and as he helped cut down the dense jungles to build the vast concrete structure on a tributary of the Mekong river in Laos, he would see US B-52 bombers heading east to drop napalm, agent orange and cluster bombs on neighbouring Vietnam, Cambodia and Laos itself.

More than two million tonnes of bombs – “one tonne for every Lao citizen”, says Vanthong – fell on the impoverished, mountainous country between 1964 and 1973. But his priority was the dam and not the “American war”, as he called it.

With $28m (£18m) aid from the western powers, he and his colleagues blasted 500,000 tonnes of rock, moved 100,000 cubic metres of soil, poured 350,000 cubic metres of concrete and drowned nearly 25 sq km (10 sq miles) of tropical forest.