AT HUAY XAI on the Thailand-Laos border, around 1,000km downriver from Mr Guo’s house in Cizhong, a sampan’s tubercular engine kicks in with a wheeze, a gag and a violent sneeze of black smoke. As the motor stammers a tiny conductor terrorises the boat, calling brusquely for tickets, chastising people for where and how they sit, shouting at the pilot to get a move on. As the boat pulls away she jumps off, smoking and yelling the whole time. Some of the locals immediately set about the business of napping, using rice sacks as pillows. Others spread bolts of fabric to picnic on, pulling out plastic bags of grilled chicken, sticky rice, bamboo shoots and tiny, floral South-East Asian oranges. The tourists, meanwhile, open cans of beer—except for the Brits, who open bottles of rotgut Thai whiskey. Tourists and locals alike start the journey staring at their phones. Eventually reception wanes and they are forced to gaze out at the wonder on all sides.

By the time it reaches Huay Xai the Mekong has already run more than the entire length of the Rhine and descended more than 3,000 metres—handily more than the Rhine manages. It still has almost two Rhine-lengths to go—but along that journey it will only drop a further 400 metres. It is a lowland river now, lush and steady. The evening chill of Mr Guo’s kitchen has been replaced by ripe, vaguely fetid South-East Asian warmth.

About two hours into the two-day journey east from Huay Xai to Luang Prabang, the boat pulls over to a muddy shore, distinguished only by a concrete staircase cut into the side of a mountain. As the boat approaches people suddenly appear out of the jungle at the top of the stairs. The scene repeats itself with minor variations throughout the journey: a muddy path rather than a staircase, a rickety little jetty or two. A couple of hours before the boat pulls in to regally sleepy Luang Prabang, it passes the Pak Ou caves: two caverns in a mountainside from which hundreds of Buddha statues brought by devotees stare down at passing boats.

Just downstream from Luang Prabang, the $3.5 billion, 1.3GW Xayaburi dam is rising—one of nine this country of 6.8m mostly rural, mostly poor people plans for the Mekong mainstream. Laos wants hydropower to be its main source of revenue by 2025: it plans to sell its capacity to neighbouring countries. The Thai government has agreed to buy 95% of Xayaburi’s power. Six Thai banks have financed the dam and a Thai construction firm is building it.

Laos’s downstream neighbours are far less enthusiastic. Cambodia (two dams of its own planned) and Vietnam (no suitable sites for dams at all), worried about the impacts that the Xayaburi dam will have on fisheries and water flow, have lodged objections with the Mekong River Commission that was set up by Cambodia, Laos, Thailand and Vietnam in 1995. The MRC is meant to facilitate co-operation among the countries that share the watershed; China and Myanmar have the status of “dialogue partners”.

Environmental groups warn that by turning a free-flowing river into a series of reservoirs the upstream Lao dams could wipe out the Mekong’s two largest freshwater species: the giant catfish and the giant pangasius. Similar warnings have been raised about the Don Sahong dam—which would span the Mekong across the breathtakingly beautiful Si Phan Don (Four Thousand Islands) region near Laos’s border with Cambodia—and about Cambodia’s giant Sambor dam, which may destroy one of the last remaining habitats of the Irrawaddy dolphin.

The Xayaburi dam’s builders have redesigned its sluice gates to allow more nutrient-rich sediment to flow downstream, and widened its fish pass to accommodate more fish of different species. The Lao government says these tweaks will alleviate the worst harms. Environmentalists are less certain. Fish passes in dams have been repeatedly found not to live up to promises made for them, particularly when fish must pass through multiple sets, as they would if Laos builds all its planned dams.

The Mekong might survive a few big mainstream dams, but a dozen—plus dozens more on its tributaries—present a qualitatively different sort of threat. This highlights a recurring theme of Mekong development: dam-builders tend to assess the impact of each dam individually; NGOs pay more attention to the cumulative effect of multiple dams. The NGOs also worry that with every new dam built, further dam-building becomes more normal, and the next project thus becomes easier to justify.

Environmentalists think both that such synergies make the harm done by dams greater than governments claim, and that the benefits are overestimated. Touting hydropower as “green” because it can be used in the place of fossil-fuel derived electricity overlooks external costs such as compensation and relocation, lost agricultural productivity and biodiversity and lowered water quality. And although benefits may be large (especially for electricity exporters), they are hardly overwhelming, especially in the context of broader development. Power demand in the region is expected to more than double between now and 2025—having already doubled from 2005. According to Richard Cronin, a Mekong specialist at the Stimson Centre, an American think-tank, the nine Lao and two Cambodian dams currently discussed would provide just 6-8% of the total electricity needs of the lower Mekong basin by 2030, with most of the power going to Thailand. “For that,” Mr Cronin asks, “you’re going to kill the river?”

Pressure from downstream countries and international NGOs has slowed Laos’s progress on its next two dams, the Don Sahong and the Pak Beng, and has forced developers to spend more money studying potential downstream harms. But Laos is a poor country with few natural resources that sees hydropower as its route to development. In the absence of better options, concerned citizens and the governments of the downstream neighbours may be able to do little more than delay Laos.

For now the Mekong remains blissfully unobstructed as it passes from Laos into Cambodia—except for the bloom of tiny islands that give Si Phan Don its name. People live on just a few of these islands. Some cater to tourists happy to spend a few days rafting the occasional rapids and listening to the Mekong rush past. But many are little more than a clump of rock and mud on which sprays of rivergrass and shrubs have taken root, still green against flowing brown. Past the waterfalls roiling the water into spume the river relaxes again. On clear days it becomes a hazy mirror, the stark blue above turning to indistinct brightness below.