Satellite altimetry reveals intensifying global river water level variability

River water levels (RWLs) are fundamental to hydrology, water resource management, and disaster mitigation, yet the majority of the world's rivers remain ungauged. Here, using 46,993 virtual stations from Sentinel-3A/B altimetry (2016‒2024), we present a global assessment of RWL variability. We find a median global fluctuation of 3.76 m, with pronounced spatial patterns: significant RWL declines across Central North/South America and Western Siberia, and increases across Africa, Oceania, Eastern/Southern Asia, and Northwestern/Central Europe. Seasonality is intensifying in 68% of basins, as high RWLs become more temporally concentrated. Maximum RWLs are declining by 0.88 cm/yr, while minimum RWLs are rising by 1.43 cm/yr. This convergence is reducing seasonal amplitude globally, with the most pronounced changes in the Americas and Central Africa. These shifts coincide with a recent surge in extreme RWL events, particularly after 2021, signaling growing hydrological instability amid concurrent droughts and floods. Our findings underscore the urgent need for adaptive water management in response to accelerating climate pressures.