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Flying rivers are long-lasting pathways of atmospheric moisture, not streams flowing through fixed channels in the sky. A 2026 study mapped these pathways over South America and found drainage patterns organized into four regional types, resembling the way rivers collect and convey water across land. It identified two major systems: a larger tropical pathway from northeastern South America toward Paraguay and southern Brazil, and a smaller temperate pathway from Patagonia toward the La Plata Basin.
What are flying rivers?
“Flying rivers” is a metaphor for water vapor carried through the atmosphere by winds. Moisture evaporates from oceans and land, travels through the air, and can later fall as precipitation hundreds or thousands of kilometers away. The study uses “aerial rivers” for persistent, long-term preferential pathways of this moisture across the continent.
These pathways are not literal rivers or permanent channels. They are also different from short-lived atmospheric rivers, which are weather systems that operate over hours to a few days. The resemblance to land drainage networks refers to the organization of moisture flows and the sequence of regions they pass through—not to water flowing in solid channels.
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Wei Weng and colleagues used a moisture-tracking algorithm driven by observation-based climate data to map long-term moisture flows. They analyzed 724 grid cells, each 1.5° × 1.5° — Wei Weng et al., Nature Communications, 2026. The authors grouped the resulting moisture-drainage patterns into four classes: headwater, drainage, outfall, and plain regions.
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Four types of aerial-river region
The labels describe positions or roles in the study’s regional drainage patterns. They are classifications of atmospheric moisture flow, not names for surface watersheds or administrative areas.
- Headwater: a source-side region in the aerial drainage sequence.
- Drainage: a region through which moisture pathways are organized as they move downwind.
- Outfall: a downstream region in the sequence.
- Plain: the fourth class identified by the study’s curve-based classification.
Two continental systems
| System | Geography and scale | Circulation alignment |
|---|---|---|
| Tropical | The larger system, originating near the continental northeast and extending toward Paraguay and southern Brazil. | Generally aligned with austral Hadley-cell wind regimes. |
| Temperate | The smaller system, running from Patagonia toward the La Plata Basin. | Generally aligned with austral Ferrel-cell wind regimes. |
These are broad patterns, not boundaries that should be treated as universally fixed. Regional moisture pathways and terrain also influence transport. The paper reports sharp transitions in the tropical system between 5°S and 13°S, as well as another transition in the Salado River Basin; these are findings about the mapped pattern, not forecasts.
How researchers identify a critical upwind source area
For a target region, a moisture-drainage curve represents how precipitation moisture received there is attributed to areas upwind. As the source area expands, additional land contributes moisture, but the rate of added contribution can change. The study uses a turning point—where the expansion yields progressively less efficient additional moisture contribution—as a criterion for delineating a critical upwind basin.
The turning point is not one continental cutoff that applies everywhere. Its location varies substantially across South America, which is why the authors argue that a single fixed threshold cannot characterize all aerial-river systems.
How aerial drainage relates to surface watersheds
Atmospheric moisture routes do not necessarily follow the boundaries of rivers or surface watersheds. The study’s examples of the Amazon and La Plata basins illustrate two different relationships:
- Amazon: aerial and surface river systems can reinforce the long-term hydrological cycle.
- La Plata: critical atmospheric source areas can lie outside the surface basin, limiting the degree of within-basin circulation.
That mismatch matters because moisture can cross watershed and national boundaries before it falls as rain. Land-use change in an upwind area can therefore affect rainfall and water availability far downwind. Identifying region-specific source areas could help inform conservation and water-resource planning, but the study does not establish a quantified rainfall increase from any particular intervention.
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Why the finding matters
Surface watershed maps show where water flows after it reaches the ground. Aerial-river mapping adds a different view: where moisture comes from before precipitation and which upwind areas are especially relevant to a downwind region. Since the critical source area differs geographically, planning based only on surface boundaries may miss important atmospheric connections.
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The study offers a method for defining those connections and comparing moisture-drainage patterns across South America. It does not make the atmospheric pathways fixed or imply that one of the two continental systems is always more important; their relevance depends on the target region and season.
Study and source
The paper, “Hydrological regimes and drainage systems of aerial rivers across South America,” by Wei Weng et al., was published in Nature Communications in 2026, volume 17, article 9341. Read the paper. A publication summary was authored by National Taiwan University and published by Phys.org on 5 October 2026: ‘Flying rivers’ above South America form drainage networks resembling those on land.
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