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.

The 34-person research team, which includes experts from McGill University, World Wildlife Fund, and several other institutions, analyzed these factors across the cumulative 7.5 million miles of rivers on the planet, in what they believe is the first global assessment of river connectivity. It took about a decade to map all the dam locations, which sometimes involved scrolling through Google Earth and manually plotting where the structures were. The analysis also considered reservoirs, levees, streets, water diversions, and other river-adjacent development.

They found that 37 percent of rivers longer than 621 miles are fully free-flowing—which means 63 percent are not. The 246 long waterways are a small minority of all earthly rivers, but their combined volume is 41 percent of the total volume of the world’s rivers, making them an important freshwater resource. Free-flowing rivers also support fish and other aquatic life, as well as adjacent wetland plants and animals. They carry sediment to deltas and beaches; without this input, these areas erode away. In some places, they support vital inland fisheries; the Irrawaddy and Salween rivers of Southeast Asia supply more than 1.2 million metric tons of food annually.

And, of these 246 very long rivers, only 23 percent flow unimpeded into the ocean (and some of the long rivers included in the 37 percent figure flow into other rivers, never quite meeting the ocean themselves). “You could argue the number [of free-flowing rivers] is even bleaker than one-third, because the really important ones go into the ocean,” says Lehner.

Dams are to blame for about two-thirds of chopped up river reaches. Flow-regulating infrastructure, like levees, and reservoirs are also affecting connectivity in a lot of the globe’s rivers. And no size stream is safe: Rivers 310 miles long or more are nearly all disconnected in the United States, Mexico, Europe, and the Middle East. In the lower 48, this includes the Mississippi, Colorado and the Rio Grande, each riddled with human development. The last long, free-flowing rivers are concentrated in remote areas of the Arctic, Amazon and Congo basins.

Looking at rivers of all lengths, the scientists found that half of all river reaches were in some way disconnected. Even a single, small dam can pose a problem. “If you’re a migratory fish it’s this one [disconnected] piece that disallows you from moving through and that’s why we declare the entire river afterwards as non-free-flowing,” says Lehner. In shorter rivers, the study probably underestimated the number of obstructions. The analysis included around 20,000 dams, just a fraction of those known.

The comprehensive map will serve many purposes. It’s an important baseline of river connectivity worldwide and will make it easier to track connectivity in the future. The findings can also help restore and protect rivers. In choosing where to build a hydropower dam, for example, developers could pick rivers that are already less connected and protect those are still free-flowing against the odds. The data could also guide efforts to remove existing dams and restore the path of ancient rivers. Importantly, protecting free-flowing rivers can also make species more resilient to climate change. As temperatures warm, the ideal range of fish and other aquatic creatures will shift, and free-flowing rivers allow them to move and adapt to these changing conditions.