Photo of the Sault Brénaz Dam on the Rhone River in France
The Sault-Brénaz Dam is a hydroelectric facility on the Rhône River in eastern France. Credit: Benoît Prieur, CC BY-SA 4.0

Dams and locks dot many of the world’s waterways, effectively fragmenting river systems. Researchers now have conducted a global survey of large rivers and found over 35,000 of these obstructions. This inventory can be used to better understand a landscape’s ecology and hydrology, the research team suggests. That’s important because riverine structures impede the migration routes of animals like fish and limit a river’s ability to transport sediments.

These results were presented in December at AGU’s Fall Meeting 2019 in San Francisco, Calif.

Things Humans Have Built

“A lot of the really important variations along rivers are related to things that humans have built,” said Tamlin Pavelsky, a hydrologist at the University of North Carolina at Chapel Hill.

Dams, locks, and other obstructions alter how water, sediments, and animals move through river systems. Last year, a study reported that only about a third of the world’s largest rivers remain completely unobstructed. (Not surprisingly, many of these pristine waterways are located in relatively unpopulated areas like the Arctic and remote parts of Africa.)

River obstructions worldwide are largely clustered in highly developed areas in North America, Europe, and South and East Asia. Credit: Global River Widths from Landsat Database and Global River Obstruction Database project members

Next year, NASA’s Surface Water and Ocean Topography (SWOT) satellite mission will begin conducting a global survey of the planet’s surface water. Knowing where river obstructions are will allow scientists to use these new data to study how man-made structures affect a river’s flow, said Pavelsky, who is the lead hydrologist for the SWOT mission. “It’s going to make our flood models a lot more accurate.”

Since 2018, Pavelsky has been leading a team of roughly 15 researchers, students, and citizen scientists mapping human-built obstructions on large rivers. The collaboration has used satellite imagery contained in Google Maps to manually identify over 35,000 obstructions located on rivers wider than 30 meters contained in the Global River Widths from Landsat Database. The team combed through imagery of roughly 2.1 million kilometers of rivers.

Fragmented Rivers in Europe

The team’s thorough searching even revealed obstructions that weren’t on maps.

The inventory the team produced includes several different kinds of dams, in addition to locks and structures that couldn’t be positively identified. The team’s thorough searching even revealed obstructions that weren’t on maps. “Maybe they were old structures that weren’t really used for anything anymore,” said Pavelsky.

The scientists found that 9 out of 10 of the world’s most obstructed rivers are located in Europe. “Rivers in most European countries are heavily fragmented,” said Xiao Yang, a hydrologist at the University of North Carolina at Chapel Hill who developed the user interface for the project. France in particular stood out with the highest density of obstructions: one every 5 kilometers, on average.

That’s not surprising, given that Europe’s rivers have been integral to communities for hundreds of years, said Yang. “There is a really long history of people building infrastructure on rivers in Europe.”

The spatial distribution of obstructions along rivers demonstrates how communities along them make use of available freshwater resources, which are often based on the geology, geomorphology, and dominant economy of the local region, said Yang. For example, because the Mississippi in the United States is a major waterway transportation route, there aren’t many structures completely obstructing it.

This inventory is one of the most extensive global databases of river obstructions, said Bernhard Lehner, a hydrologist at McGill University in Montreal, Que., not involved in the research. It’s “a very valuable and unique contribution.”

Going Smaller

“We’re potentially providing a really good training data set for machine learning.”

In the future, Pavelsky and his colleagues want to expand their technique to include smaller rivers. “From an ecological or geomorphological perspective, it’s at least as important to understand what’s going on on smaller rivers,” said Pavelsky. The team is also interested in collaborating with groups developing automated search algorithms to detect obstructions. “We’re potentially providing a really good training data set for machine learning,” said Yang.

—Katherine Kornei (; @katherinekornei), Freelance Science Journalist


Kornei, K. (2020), Europe’s rivers are the most obstructed on Earth, Eos, 101, Published on 31 January 2020.

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