Hidden Crisis in Off‐Stream Reservoirs From Their High Sedimentation and Ageing Risks

Abstract Aging dams and reservoir sedimentation pose a growing threat globally. While large dams on streams dominate scientific attention, the small, especially off‐stream reservoirs are vital to rural communities but receive disproportionately less attention. This study first quantifies the proportion of off‐stream reservoirs in the United States (58%), Europe (38%), and China (70%), indicating their substantial presence worldwide. We then focused on the densely dammed Upper Yangtze River Basin, where we compiled a comprehensive data set of 13,886 reservoirs and applied an enhanced sediment model to estimate cumulative sedimentation. Although on‐stream mega dams dominate the control of basin‐wide sediment flux, off‐stream reservoirs play a critical but overlooked role in local sedimentation and safety. Results show that while on‐stream large reservoirs (storage capacity ≥100 Mm 3 ) dominate total storage (>97%) and trap the vast majority of sediment, off‐stream reservoirs account for over 75% of all reservoirs and exhibit a markedly higher average sedimentation ratio (11.3%) than the basin average (3.3%) and that of on‐stream large reservoirs (2.8%). By 2050, 137 reservoirs will exceed 100 years of age, 87% of which are off‐stream. Alarmingly, 20% of these aging off‐stream reservoirs also face severe sedimentation (sedimentation ratio ≥50%), representing an extreme failure risk. Our findings reveal a hidden crisis that off‐stream reservoirs, despite their numerical dominance, face more acute aging and sedimentation challenges and are largely overlooked in current dam safety frameworks, highlighting an urgent need to integrate them into sustainable water infrastructure management.