Wide-swath Altimetry Observes Global River Water Storage Variability

Rivers are Earth's most renewable and accessible freshwater resource[1], yet global estimates of the magnitude and variability in river water storage have remained few and inconsistent[1–9]. Previous estimates of variability have relied either on sparse and asynchronous remote sensing observations[10] or on hydrological models constrained by incomplete understanding of surface water balance and poorly known river channel characteristics[2,3]. The insufficient knowledge of temporal variations in river water storage across space hinders effective management of this critical freshwater resource[11,12]. Here we present the first global observations of active river channel geometry and associated monthly changes in water storage at the reach scale derived from the first water year (October 2023 to September 2024) of the Surface Water and Ocean Topography (SWOT) mission at 126,674 reaches worldwide. Clear patterns of riverbed shape and storage variability expectedly emerge across major basins. SWOT reveals a range of 321.6 km³ in global annual river storage variability, approximately 26% lower than the lowest previously modeled estimates for the same wide reaches. While the Amazon’s 2024 record drought likely contributed to the discrepancy, the new observations point to distinct knowledge limitations in surface water science. These findings highlight key opportunities to improve the fundamental representation of surface water dynamics in global models and to better inform water resource management and disasters mitigation at scale.