ON THE evening of July 23rd villagers along the Xe Pian river in southern Laos heard the roar of surging water. A dam 5km upstream had collapsed. The torrent inundated six villages. Homes were swept away, and around 7,000 people displaced. At least 20 are known to have died. Rescue attempts are being hampered by poor phone signals and heavy rains.

The dam was under construction when it burst. Fractures were discovered the day before, according to SK Engineering & Construction, a South Korean firm in the $1.2bn hydroelectric venture of which the dam forms a part. Rain had hindered attempts to fix the damage.

The disaster highlights Laos’s big bet on hydroelectric power. The communist country is landlocked and cash-strapped. Its solution is to build lots of dams along the Mekong river and its tributaries, with Thai and Chinese backing, and to sell the electricity to neighbours. Last year Laos had 47 projects. It plans to have 100 by 2020. Laos wants to become the “battery of Asia”. In 2017 electricity accounted for 23% of its exports, up from 12% in 2010.

The strategy has costs. The dams turn a free-flowing river into a chain of reservoirs, says Maureen Harris of International Rivers, an NGO. That makes it hard for migratory fish to breed, especially the giant catfish and the giant pangasius, two of the Mekong’s largest freshwater species. The river also carries nutrient-rich sediments to rice farmers downstream in the Mekong delta in Vietnam. A recent study by the Mekong River Commission estimated that if Laos achieves its dam-building target, the amount of sediment that reaches the delta will fall to only two-thirds of its level in 2007.

Now, to the list of concerns, add the safety of locals. This week’s flood is not the first to be caused by such a collapse. In September a dam in Xiangkhouang in the north broke. Happily, no one died then. But if the country’s hydroelectric ambitions make flooding worse, then calls to power down the battery of Asia will grow louder.