The SWOT satellite for the first time measures globally how much the world’s rivers “swell” and “drain” over the year
River flows pulse through the seasons: they rise after rains and snowmelt, and fall in the dry months. But how much does the amount of water in river channels actually change, and how reliable have previous estimates been? A new analysis of the first “water year” of data from the Surface Water and Ocean Topography (SWOT) satellite— a joint mission of NASA and the French space agency CNES—brings the first global, observational insight into that question, with a surprising conclusion: the total annual oscillations of river water storage appear smaller than earlier models suggested.
SWOT was launched in December 2022 to measure, with high precision, the height and spatial extent of surface waters—not only oceans, but also most lakes and rivers on Earth. The key difference from earlier satellite methods is the simultaneous measurement of multiple water dimensions: width, water-surface height, and slope, enabling finer tracking of changes in time and space. This is achieved by the Ka-band Radar Interferometer (KaRIn) instrument, which reconstructs water surfaces across a wide swath along the satellite’s ground track by “bouncing” microwave pulses off the water surface and measuring the signal return time.
What is new compared with previous estimates
For decades, hydrology has relied on combinations of field measurements (river gauges for discharge and water level) and modeling, especially in areas where the gauging network is sparse or access is difficult. Under such conditions, scientists often had to merge different sources: satellite altimeters for water-surface height and optical or radar imagery for river width. The problem is that these measurements do not necessarily occur at the same time and do not cover all parts of the world equally, so in practice one still relied on models and assumptions about channel geometry.
SWOT, thanks to wide-swath altimetry, provides river width and water-surface height simultaneously for a large number of river segments. According to data from a research preprint analysis describing global “mapping” of bank shapes and changes in river water storage, this includes approximately 126,674 river reaches observed from October 2023 to September 2024—during the first complete hydrological cycle after the initial instrument calibration and validation phase. This global, uniform picture opens the possibility of comparing major river systems in a way that has not been feasible until now.
Smaller global oscillations—and why the Amazon matters in that calculation
In aggregate, global annual variability of river water storage in that period is estimated at about 313.4 cubic kilometers (km³), which is roughly 28% less than the lowest earlier model-based estimates for comparable wide and measurable river reaches. The researchers emphasize that an intense drought in the Amazon—one of the planet’s key hydrographic “heavyweights”—likely influenced the overall result.
The Amazon has the world’s largest river volume, and during dry episodes its hydrology changes in a way that globally “pulls” the statistics. The drought in central Amazonia in October 2023 led to record-low water levels on tributaries such as the Rio Negro, with widely documented impacts on local communities, transportation, and ecosystems. NOAA’s climate service describes how months-long rainfall deficits and extremely low water levels during that period were among the most pronounced in long measurement records. In that context, it is not hard to understand why the observation period (October 2023–September 2024) could yield a “drier” picture of global variability than would be the case in a climatologically average year.
At the same time, the fact that even under drought SWOT records the largest annual oscillations precisely in the Amazon suggests a double message: on one hand it confirms that the satellite captures real changes in the largest systems, and on the other it reminds us that global averages can shift strongly depending on what kind of year we observe.
The Nile: an unexpectedly “calmer” signal and the limits of the first year
One of the more interesting findings concerns the Nile. In public summaries and interpretations of SWOT data, it is often highlighted that the Nile—the longest river in the world—showed less variability than would be expected from some earlier estimates. Possible explanations range from the influence of dams and upstream regulation systems, through dry conditions in certain years, to methodological challenges that accompany introducing a completely new satellite technique into operational science. It is important, however, to distinguish two things: a real hydrological stabilization of flow due to water management (damming, reservoirs, regulation) and statistical effects that arise when the instrument and algorithms are still “learning” the hardest parts of the task—for example, complex channel geometries, vegetation-covered banks, or areas strongly affected by tidal waves and backwater effects.
That is precisely why scientists treat this set of results as a baseline. SWOT is now in its operational phase, and the mission’s value grows as the observation record lengthens: more years mean a better separation between “weather” (a single dry or wet year) and “climate” (long-term patterns).
From an “invisible” channel to a bank map: how SWOT reveals the shape of river channels
Beyond volume and water level, SWOT opens another area that has been poorly mapped for years: the underwater topography of river channels—i.e., the shapes of banks and beds that determine how water moves and where it will “spill” during high water. Field surveys of channels are expensive and logistically demanding, and on many rivers practically impossible due to remoteness, safety reasons, or political constraints.
Wide-swath altimetry, combined with changes in water level over time, makes it possible to reconstruct patterns from a series of “cross-sections”: whether the channel is concave or convex, steep or gentle, stable or highly variable. Such information is not academic exotica. It directly feeds into flood models, planning of river transport and navigability, assessments of erosion and sediment deposition, and understanding habitats of river ecosystems.
Why these numbers matter for public policy and the economy
The question “how much water rivers hold” may sound abstract at first glance, but in practice it translates into very concrete decisions. Water-resource management requires reliable data on seasonal storage, especially in areas where water is shared among agriculture, energy, industry, and households. In years of extreme drought, such as the Amazon episode of 2023, falling water levels can halt river logistics, endanger drinking-water supply, and reduce hydropower production. In years of extreme rainfall, the same channel and bank geometry determines how quickly a flood wave will arrive and how much energy will be “dissipated” across floodplains.
A broader frame is also provided by the international climate community. The World Meteorological Organization (WMO), in its recent summaries of hydrological extremes, warns that in recent years many river basins have been outside “normal” conditions, with more frequent and more intense droughts and floods. In such a world, satellite monitoring that can “close gaps” where there are no gauges becomes an infrastructural necessity, not a luxury.
How SWOT measures—and what it (still) cannot say
It is important to emphasize limitations to avoid false expectations. SWOT does not “see” directly the absolute volume of water in an entire river as if it were a measuring cup. It measures the geometry of the water surface—height, width, and slope—and from that, with appropriate models and assumptions about channel shape, changes in water storage in the active channel are estimated. That means precision can vary from river to river, depending on how simple or complex the channel is, how “noisy” the environment is for radar due to vegetation, and how large the influence of floodplains outside the main channel is.
Still, KaRIn as an operational instrument, according to the NASA Earthdata description, is designed precisely to measure water-surface height across a wide swath, making SWOT unique compared with previous nadir altimeters that “cut” the planet with a thin line. In practice, that means that with the same overflights one gets more information and a better spatial picture—crucial for rivers, which are narrow, winding, and often hidden under clouds or tree canopies when optical methods are used.
What’s next: longer records, faster products, and applications in early warning
As the mission continues, SWOT data are expected to increasingly enter operational systems, from scientific studies to risk-management applications. Earlier NASA work with SWOT has already shown that the satellite can “capture” large river waves caused by extreme rainfall and other events, opening space for better understanding the dynamics of flood waves and their downstream propagation. This is especially important in poorly instrumented basins, where warnings often arrive late or are based on rough estimates.
Scientifically, the biggest gain could be in what is called “closing the budget”: how much rainfall and melted snow ends up in rivers, how much water remains in soils and groundwater, and how much returns to the atmosphere. Rivers are the visible outlet of that entire system, but until now the global picture has been assembled from fragments. SWOT for the first time offers the possibility to connect those fragments into a more consistent map, in which both the largest and remote river systems are observed with the same “yardstick”.
Sources:- Research Square (preprint) – global analysis of SWOT data for 126,674 river reaches, variability 313.4 km³ and comparison with models (link)- NASA Earthdata – description of the Ka-band Radar Interferometer (KaRIn) instrument and its role in measuring water-surface height (link)- NOAA Climate.gov – overview of drought and record-low water levels in central Amazonia in October 2023 (link)- NASA – SWOT mission and an example application in detecting large river waves (flood/flow waves) (link)
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