“The benefits of free-flowing rivers are countless,” Denielle Perry, a water resources geographer at Northern Arizona University in Flagstaff who was not involved in the study, told Stefan Lovgren at National Geographic . “Rivers are the lifeblood of the planet.”

About two-thirds of the world’s longest rivers are no longer free-flowing, thanks to damming, diversions, and other human-made disruptions, according to new research . The findings , published today in the journal Nature, spell trouble for people and wildlife worldwide who rely on the natural paths of rivers for water, food, irrigation, and more.

The most common obstructions to waterways were dams, of which there are about 2.8 million globally. Activities like reservoir construction and sediment trapping were also found to reduce or alter river flow.

By analyzing aerial and satellite images, a team of international researchers constructed a comprehensive portrait of rivers worldwide, mapping 7.5 million miles of waterways both impeded and free-flowing. To qualify as free-flowing, a river had to score highly on the researchers’ “connectivity” index, which measured how human infrastructure had modified the water and its surroundings. Of the world’s shortest rivers—those fewer than 62 miles long—97 percent had a high connectivity score. But most of the world’s longest rivers, those spanning more than 620 miles, didn’t make the cut, with 63 percent blocked off or otherwise altered.

Human impact on rivers, however, isn’t distributed evenly. A few untouched waterways remain in less populous areas, such as the Arctic, the Amazon, and the Congo Basin. But in places like the United States, China, and western Europe, most major rivers have been fragmented under human influence—and there are at least 3,700 new large dams currently in the works worldwide.

Even more extreme results have been reported by another recent study, which conducted a more fine-grained analysis of waterways in the United Kingdom and suggests human-made structures have interrupted 97 percent of Great Britain’s river network. Unlike the Nature study, which was global, the British study accounted for the effects of very small dams. “We believe free-flowing rivers simply don’t exist anymore, at least in Europe,” Swansea University’s Carlos Garcia de Leaniz, who led the UK study, told Damian Carrington at The Guardian.

Humans have gained much by diverting rivers from their natural courses. Modified waterways have allowed entire civilizations to spring up on shorelines. Dams around the world help maintain a steady supply of water, and, in some cases, electricity.

However, when humans reroute rivers, they inevitably deprive ecosystems of vital resources. Diversions and dams can also increase flooding in some regions, displacing populations of humans and wildlife alike. Due in part to habitat loss, freshwater plant and animal species are now declining much faster than those on land.

These declines in biodiversity can have knock-on effects for our own wellbeing: As fish and other animals disappear from the world’s fractured rivers, food security is likely to be imperiled in many parts of the world, Herman Wanningen, an aquatic ecologist at the World Fish Migration Foundation in Holland, told Lovgren at National Geographic. “When a dam is put in, the free-flowing river suddenly becomes a stagnant reservoir, the natural habitat disappears, and with it the fish.”

And these consequences aren’t just hypothetical: Dams in the Columbia River in the United States and the Yangtze River in China have led to crashes in salmon and paddlefish populations, respectively, Lovgren reports.

The Tahoma Dam in North Carolina. Image Credit: Joshua Moore, iStock

In light of these findings, some experts advocate for choosing wind or solar power over hydropower. “While hydropower inevitably has a role to play in the renewable energy landscape, well-planned wind and solar energy can be more viable options for rivers and the communities and biodiversity that rely on them,” study author Michele Thieme told Carrington at The Guardian.

In the meantime, several countries have already recognized the importance of keeping rivers intact—including the United States. Though the country is still home to more than 80,000 dams of all sizes, in the past several decades, several river restoration projects have taken hold, some of which have even led to the rehabilitation of salmon and trout species once lost to the Pacific Northwest, Lovgren reports.

With this new information in hand, policymakers might be able to weigh the pluses and minuses of maintaining certain structures, the authors say. It’s unclear what the future holds, but the dams of the future might encounter barriers themselves.