Satellite reveals a world of ‘weird rivers’ shaped by humans

“The river is a river, not a line,” sang folk singer Patty Griffin. After nearly 4 years in orbit, a radar satellite launched by NASA and CNES, the French space agency, is revealing the truth of the lyric. By simultaneously measuring the water levels and breadth of rivers around the world, the Surface Water and Ocean Topography (SWOT) satellite is exposing the true complexity of those blue lines on a map. They are dammed, braided, frozen, or drained—with behaviors that hydrologic models struggle to capture, says Colin Gleason, a hydrologist at the University of Massachusetts Amherst who led a study, published last month in Geophysical Research Letters, which found that the vast majority of rivers are not simple, idealized channels. “Human-affected weird rivers dominate the global riverscape.” Before SWOT’s launch in late 2022, scientists had no way to routinely monitor the planet’s rivers. Gauges measure depth in real time at particular locations, but they are sparse, especially in less developed countries. Satellites could observe a river’s width or water level, but not both at the same time. To fill the gaps, researchers relied on models, increasingly informed by artificial intelligence trained on these spotty data. To predict whether a river will dry up in a hot summer or flood after a heavy rain, the models make simple assumptions about the shape of its channel and how much water reaches it. SWOT is poised to revolutionize these models. The $1.2 billion satellite revisits most spots on Earth at 11-day intervals, mapping rivers and lakes as small as 30 meters wide. It broadcasts radar pulses from its belly, and two antennas separated by 10 meters catch the reflections, viewing the same patch of water from slightly different angles. Comparing the returning radar signals allows SWOT to determine the elevation of the water surface while also mapping its extent. Its accuracy and resolution have exceeded expectations, says Tamlin Pavelsky, a hydrologist at the University of North Carolina at Chapel Hill and co-lead of SWOT’s freshwater science team. “SWOT has blown our minds,” he says. “If you told us before launch it was as good as it is, I would have thought you were lying.” In the new study, Gleason and his colleagues examined some 68,000 rivers, comparing SWOT data with existing hydrologic models. They found the models struggle to capture the behavior of many rivers, especially in China, the Arctic, and in arid regions. In some cases, models predicted rising river waters when SWOT in fact saw them falling. “This gives us an atlas of where our basic knowledge of rivers may be lacking,” Gleason says. Gleason says the study isn’t meant to undermine river models so much as show how SWOT could improve them. All of the major modeling groups are co-authors, and many are now busy integrating SWOT’s measurements into their work. “SWOT is a catalyst for rebuilding global hydrology from the ground up,” Gleason says. “I hope we get there.” One early use of SWOT has been to improve the global flood models that insurance companies and policymakers rely on to project hazards. Building these global models, “we have to make a number of nasty assumptions about water surface heights,” says Jeffrey Neal, a hydrologist at the University of Bristol. In a study published in Water Resources Research in July, Neal and his co-authors showed a flood model built with SWOT data could be as accurate as one informed by sonar mapping of a riverbed—data that are scarce and expensive to collect. Fathom, a division of the reinsurance giant Swiss Re, is now wiring SWOT data into its global flood model. Neal, meanwhile, is applying the technique to forecast flood risks in southern Africa as climate change alters rainfall there. SWOT is also revealing processes that scientists could previously observe only on rivers instrumented with gauges. One example is flow waves: long-lived surges of water that propagate downstream, generating flash floods. Last year, work led by Hana Thurman, a graduate student at Virginia Tech, showed SWOT could capture them, although the processing needed means it cannot provide real-time warnings. “People are seeing a lot more how river waves work,” Thurman says. She now hopes to probe why some rivers are prone to these dangerous waves and others aren’t—a topic that is all the more relevant after the river flooding disasters last year in Texas and this year in Nepal. The satellite can also see waves moving in the opposite direction, upstream, pushed by the ocean’s tides. In a study published in Nature earlier this year, Michael Hart-Davis, an oceanographer at the Technical University of Munich, and his colleagues used SWOT to look at more than 51,000 river branches. They identified more than 165,000 kilometers of rivers where tides intruded, exposing local ecosystems to brackish water. In some poorly monitored rivers in South Africa and elsewhere, tides penetrated surprisingly deep, Hart-Davis says. Even on the Amazon River, where tidal influences are well-known, seeing its full extent was extraordinary, he says. “It’s a crazy tree how far up the tides go.” The same measurements are opening a window on lakes and wetlands, including Florida’s Everglades and even wetlands beneath dense tropical forests, Pavelsky says. Jida Wang, a limnologist at the University of Illinois Urbana-Champaign, is using SWOT to do a comprehensive study of more than 5 million lakes. His unpublished analysis shows the amount of water they store varies by up to 70% more than previously thought. Artificial reservoirs, where water is stored and released, drive half the variability, despite making up only 5% of total lakes. “Human regulation really tends to dominate the natural signal,” he says. That’s true in rivers as well, says Arnaud Cerbelaud, a remote-sensing hydrologist at CNES. Earlier this year, he led a paper in Nature charting 1 year of water-storage changes in the world’s rivers. He and his colleagues found that some rivers, such as the Nile, were shockingly lower than models suggested—perhaps because of irrigation demand, or the construction of a massive new dam in Ethiopia. “We are altering the flows of rivers everywhere on the planet.”