In a landmark achievement, researchers from NASA and Virginia Tech have, for the first time, used spaceborne radar to detect and measure large‐scale flood waves traveling down U.S. rivers. The Surface Water and Ocean Topography (SWOT) mission—launched in 2022 as a joint effort between NASA and France’s CNES—carries the Ka‐band Radar Interferometer (KaRIn), a cutting‐edge instrument that maps water‐surface elevation across a 30-mile (50-kilometer) swath on either side of its ground track.
Key Findings: Three Flood Waves Tracked from Orbit
• Yellowstone River Ice-Jam Surge (April 2023): SWOT observed a 9.1-foot (2.8-meter) crest flowing toward the Missouri River following an upstream ice-jam breakup.
• Colorado River Rainfall Wave (January 2024): A 30-foot (9-meter) flood wave stretching 166 miles (267 kilometers) south of Austin, Texas, traveled at 3.5 feet per second (1.07 m/s) into Matagorda Bay.
• Ocmulgee River Flood Wave (March 2024): A 20-foot (6-meter) surge extending over 100 miles (165 kilometers) in Georgia moved at about 1 ft/s (0.33 m/s).
“These observations reveal flood dynamics we’ve never seen from space,” said Dr. Cédric David, hydrologist at NASA’s Jet Propulsion Laboratory. “SWOT’s high spatial resolution allowed us to distinguish wave crests and tails over tens to hundreds of miles, a new capability for hydrology.”
How KaRIn Measures River Waves
KaRIn emits microwave pulses toward Earth’s surface and records the phase difference between two separated antennae. This interferometric technique yields two‐dimensional maps of water‐surface height with centimeter‐level accuracy. Combined with SWOT’s revisit frequency—up to twice every 21 days for a given location—the instrument can capture evolving flood waves in large rivers.
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Lead author Hana Thurman, a PhD candidate at Virginia Tech, sifted through SWOT’s global dataset to identify anomalies in river elevation indicative of moving waves. “We treated the satellite data like a flood‐event detective story,” she explained. “By comparing successive overpasses, we could track wave speed, height, and length, and then link them to triggers like extreme rain or ice‐jam release.”
Case Study: Yellowstone River Ice Jam
On April 19, 2023, SWOT passed over the Yellowstone River near Glendive, Montana, just as an ice jam upstream gave way. KaRIn recorded a sharp 2.8-meter elevation spike moving downstream. Sediment‐laden water, visible in concurrent Sentinel-2 optical imagery, confirmed the event’s timing. Thurman and colleagues divided the wave into a steep 11-kilometer crest followed by a longer, attenuating tail—information impossible to glean from fixed stream gauges alone.
Rain‐Driven Waves in Texas and Georgia
The flood wave on the Colorado River stemmed from intense rainfall on January 23–24, 2024. SWOT’s pass two days later revealed a 9-meter surge spanning 267 km. The wave’s propagation speed (≈1.1 m/s) matched USGS gauge‐based estimates, validating the satellite’s accuracy. Similarly, a 6-meter wave on the Ocmulgee River was linked to heavy rain upstream of Macon, Georgia.
Complementing Traditional Stream Gauges
Hydrologists have long depended on river‐gauge networks—toll‐booth–style measurements at discrete locations—to monitor floods. However, gauges are sparse in many basins and cannot capture full wave morphology. “SWOT fills in the gaps, providing quasi‐continuous profiles of river elevation over long reaches,” said Dr. George Allen of Virginia Tech. “It’s like trading individual speed traps for a traffic‐helicopter view of the entire highway.”
Toward a Global Flood Monitoring System
SWOT’s global coverage and ability to detect flood waves on rivers larger than roughly 100 m wide make it a powerful tool for water managers worldwide. Researchers estimate the mission will observe more than half of large‐scale floods during their lifecycle. The next steps include automating wave detection algorithms and integrating SWOT data into flood‐forecast models.
“Early identification of a flood wave’s speed and height could improve warnings for downstream communities,” Allen noted. “Imagine knowing a six-meter wave is headed your way 48 hours in advance—that’s game‐changing for emergency response.”
Challenges and Future Directions
Despite SWOT’s successes, challenges remain. The satellite revisits each location only once every 21 days, so capturing rapid flood pulses requires luck in timing. Also, KaRIn’s observations are limited to daylight overpasses in clear weather. Researchers plan to combine SWOT with geostationary radar concepts and ground‐based gauges to achieve near‐real-time monitoring.
Scientists also aim to extend river‐wave studies beyond the United States to regions with minimal in situ data, such as parts of Africa, South America, and Asia. Such efforts could illuminate how climate change is altering flood patterns in monsoon and snowmelt‐driven systems.
A New Window on River Dynamics
“Oceanographers have studied waves for centuries; now rivers are on the map,” said Dr. Nadya Vinogradova Shiffer, SWOT program scientist at NASA Headquarters. “As the planet’s arteries, rivers transport water, sediment, and nutrients. Understanding their dynamic surges not only helps mitigate flood risk but also enriches our knowledge of Earth’s hydrological cycle.”
The team’s findings, published May 14 in Geophysical Research Letters, mark the beginning of a new era in remote sensing of inland waters. With SWOT’s data and future satellite missions, scientists are poised to unlock the secrets of river waves—and to safeguard communities from the hidden hazards surging down their waterways.
