Rivers are the Earth’s arteries, and humans are clogging them with all sorts of junk. In fact, just a little over a third of the planet’s longest rivers remain free from human interference, according to a new study released in Nature on Wednesday. Like the arteries in our bodies, choking rivers could lead to a planetary health crisis.
“We see this biodiversity crisis worldwide and in rivers particularly, the decline in biodiversity is staggering and really concerning,” Günther Grill, a geographer at McGill University who led the study, told Earther.
Left to their own devices, rivers transport all sorts of good stuff around Earth, including freshwater, fish, nutrients, and sand. But humans have turned to dams to regulate those flows and generate electricity, allowing communities to spring up on once unruly banks. Levies, roads, and urbanization have further interfered with the natural course of rivers. This has largely been to the detriment of rivers and the people and ecosystems downstream (and sometimes the people living next to them, for that matter).
In an effort to suss how restricted rivers have become, Grill and an international team of scientists first set out to define what a “free-flowing” river looked like. Their definition of a free-flowing river is pretty simple: any waterways that remain connected in a way that allows ecosystems to function as they normally would. The researchers took stock of how many were left using dam data and river models.
They then analyzed an astounding 7.6 million miles of rivers—enough waterways to go to the moon and back 15 times—broken down into short, medium, long, and very long rivers. The findings show most short and medium-length rivers are still largely free-flowing, though Grill warned that the results likely underestimate dams on them.
“We are more confident looking at patterns from larger rivers,” he said.
There, the results are much more stark. Sixty three percent of very long rivers—those 620 miles or more in length—are no longer free flowing, blocked up by many of the estimated 2.8 million dams globally. Of the 37 percent of mega rivers that remain free-flowing, most are in the Arctic, Amazon, Congo, all areas that are hard to develop and lightly populated.
The loss of very long rivers running their natural course has huge impacts for downstream areas, and not just on biodiversity. Louisiana, for example, is losing land and its delta ecosystems in part because the Mississippi River that once flowed freely is now under the control of a huge network of dams. Grill pointed to loss of connectivity in the Mekong River basin of Southeast Asia, which reduces the movement of fish, and could exacerbate food security.
“In Cambodia, most protein comes from fisheries products,” he said. “The loss of fisheries to dams could have direct and negative impacts on tens of millions of people.”
Grill said he views the study as a jumping off point for future analysis and decisions. In the electricity realm, those decisions may include weighing whether hydropower is a better option than renewables. And in the ecosystem realm, they may including deciding if a dam on an already impacted river is smarter than clogging a free flowing one, or how and when to release water from dams to mimic the natural world, something water managers have done with the Colorado River in recent years.
“We need smart solutions going into the future,” Grills said. “We need to look at the bigger picture.”