Most large rivers don’t flow freely anymore

Popular Science –

Dammed up, drained, and dredged of sediment, the Earth’s rivers aren’t doing so well. And a new study shows that the number of free-flowing rivers, which move unimpeded on their route toward the ocean, is even lower than we previously thought.

Of the 246 rivers longer than 621 miles, only about a third flow freely across their entire length, according to the study, published Wednesday in the journal Nature. The rest are dammed, channelized, or otherwise heavily developed. “Our results are even worse than in the past,” says Bernhard Lehner, an author on the study and hydrologist at McGill University. “Fifteen years ago, researchers said about half of the large river basins in the world are impacted, and we now say two-thirds.”

A river’s connectivity is based on a few measures. The most obvious perhaps is how obstructed its waters are, from its source to its terminus. But the researchers also considered a river’s ability to naturally spread out into the surrounding floodplain, as well as how deep its waters can infiltrate into the ground. Seasonality is a factor, too—some rivers don’t flow year-round, and therefore aren’t truly considered free-flowing.