The banks of the Mekong River in Vientiane, the capital of Laos, can be a lovely retreat at sunset. The river sweeps alongside the city in a wide elbow curve, offering a panoramic view of tranquil waters and tree-lined shores. Thailand rests on the opposite bank, seeming farther away than its half-mile distance. And as the setting sun lights the water ablaze, birds skim the surface, and fish make themselves known with the occasional splash, making an evening walk along the riverbank a pure delight.

At the start of a recent visit to Vientiane, however, one of us (Sterling) wound her way through the city to the river, anticipating a cool breeze and a quiet walk after a sweltering workday, only to stare into a scene from the desert. Clouds of dust rose from the riverbed, where a group of kids were playing soccer. Beyond that bone-dry sandbar, a vestige of the river was just visible as a thin stream along the far bank. By all appearances, one could easily have walked across to Thailand.

Such radical fluctuations are natural to the Mekong, and whole communities—human and wild—are adapted to its periodic floods and droughts. The river swells when rainfall rushes down its tributaries and shrinks again in drier weather. But the rise and fall of the Mekong is increasingly dictated by energy use in China and Thailand. Upriver hydroelectric dams dampen the fluctuations and change the timing of floods and dry spells, affecting water-dependent wildlife hundreds of miles away. The extent of those changes is likely to grow as more dams, scheduled for construction, make their mark on the river.

The dams are just one of the many troubles that confront the river and its denizens; water extractions, pollution, invasive species, and overfishing also threaten the ecosystem’s health. And the Mekong’s woes mirror those of freshwater systems worldwide, which are increasingly pressured by a growing human population that makes ever-greater water demands. The scale is enormous: people now appropriate more than half of the world’s accessible surface freshwater, leaving precious little for natural systems and other species to thrive.

As a result, even as the human population of the globe has doubled, many species that depend on freshwater ecosystems have suffered steep declines. The list would bring tears to a conservationist’s eyes: in the past three decades, a fifth of the world’s water birds, a third of freshwater mammals, a third of amphibians, and more than half of freshwater turtles and crocodiles have become either threatened, endangered, or extinct. Freshwater fishes represent a quarter of the world’s living vertebrate species, and yet more than a third are threatened or endangered. The ecology of freshwater systems may be irreversibly damaged if we humans don’t improve the way we treat them.

The Mekong’s name translates from Lao as “mother of the waters.” It’s no wonder: the river snakes some 3,000 miles from its headwaters on the Tibetan Plateau to its outlet through the Mekong River Delta into the South China Sea. It and the uncountable “feeder” rivers and streams in Cambodia, China, Laos, Myanmar, Thailand, and Vietnam make up the 300,000-square-mile Mekong River Basin [see map above].

That mesh of waterways is one of the most productive and diverse ecosystems on Earth, supporting more than 6,000 species of vertebrates alone. Its fish fauna, with some 2,000 species, of which sixty-two are endemic, exceeds all but those of the Amazon and Congo river basins. The wetlands harbor several threatened and endangered birds and mammals, including the eastern sarus crane, Grus antigone sharpii; the Bengal florican, Houbaropsis bengalensis; and the hairy-nosed otter, Lutra sumatrana, which was recently rediscovered after having been feared extinct. Sixty-five million people live there, too, 80 percent of them dependent on the river for their livelihood as farmers and fishers.