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Clauge River in Jura, France, in nonflowing phase. Photo: Bertrand Launay/National Research Institute for Agriculture, Food and Environment

A new detailed map reveals more than half of the world’s network of rivers and streams runs dry for at least one day each year.

Why it matters: Knowing how often and for how long rivers stop flowing is important for managing water resources and conserving species adapted to these unique ecosystems, but so-called nonperennial rivers are understudied and often overlooked.

How it works: Small streams carry organic matter and nutrients to larger rivers, lakes and the ocean.

  • “They are what connects a lot of the terrestrial environment to the freshwater environment,” says Mathis Loïc Messager of McGill University and the National Research Institute for Agriculture, Food and Environment in Villeurbanne, France, who is an author of the new study.
  • Some of these rivers and streams stop flowing naturally — due to winter freezing in some places and dry seasons in others — but also from overuse for irrigation and other purposes.

What they found: Most types of rivers and streams in the world flow intermittently.

  • Messager and his colleagues got a handle on the distribution of nonperennial rivers and streams by first pairing water flow measurements from 5,615 stations around the world with data about the climate, soil, geology, land cover around the rivers and other environmental variables at those collection sites.
  • The researchers then extrapolated that model of where rivers run dry and estimated between 51% and 60% of the world's 40 million miles of mapped rivers and streams stop flowing for at least one day each year.
  • And between "44% and 53% of the global river network ceases to flow at least one month per year," they report this week in Nature.
  • Nonperennial streams and rivers were found across all climates — from parts of the Niger River in West Africa to the headwaters of the Amazon. The team estimates for "52% of the world’s population in 2020, the nearest river or stream is nonperennial."

What's next: The researchers are working on estimating how human activities, in particular agriculture and energy generation, have impacted the distribution of nonperennial rivers and how climate change may impact them in the future.

  • Climate change-fueled droughts in Australia and parts of Africa could cause more rivers there to flow intermittently in the future.
  • But it isn't just rivers drying up: Rising temperatures in the Arctic may mean rivers that typically freeze and stop flowing in the winter may not anymore, Messager says.

"Now that we know these are a really important part of the global river network and the role of rivers in the global carbon cycle, we have to catch up," Messager says.

  • There is a need for tools and protocols to monitor the health of these rivers and for water use guidelines to take into account that a river's flow can go to zero, he adds. "We need to speak in new terms."

Go deeper

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South Korean President Moon Jae-in and North Korean leader Kim Jong Un in Pyongyang, North Korea, in 2018. Photo: Pyeongyang Press Corps/Pool/Getty Images

North and South Korea's leaders have pledged to improve relations and resume previously suspended communication channels between the two countries.

Why it matters: The resumption of the hotline on Tuesday comes despite stalled negotiations between Washington and Pyongyang on the denuclearization of North Korea, which broke down after a second summit between then-President Trump and North Korean leader Kim Jong-un ended without a deal in 2019.

Updated 56 mins ago - Sports

Teen swimmer Lydia Jacoby wins 1st U.S. women's Olympic gold in Tokyo

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Team USA's 17-year-old swimmer Lydia Jacoby has won the Olympic gold medal in the women's 100-meter breaststroke at the Tokyo Games, completing the race with a time of 1:04.95.

Of note: The Alaskan beat defending Olympic champion and fellow American Lilly King, who won bronze. Tatjana Shoenmaker from South Africa took home the silver medal.

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Scoop: Pelosi’s new COVID plans

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Why it matters: The spread of the Delta variant has alarmed both members and staffers anxious about interacting with the unvaccinated. Pelosi’s anticipated move — continuing an emergency COVID-19 measure enacted last year so lawmakers could vote remotely — is aimed at allaying those concerns.