Satellite imagery reveals rapid expansion of human-regulated reservoirs globally.

Human-regulated reservoirs play a crucial role in water resource management, yet their global distribution and expansion remain poorly understood. Utilizing satellite imagery, we conducted a comprehensive analysis of reservoir dynamics from 2000 to 2018. We identified 425,023 (419,982 in 2018, 387,823 in 2000) on-channel reservoirs ≥0.01 km2. We found small reservoirs (<0.1 km2) expanded rapidly (8.1% increase in area), while large reservoirs remained relatively stable. This expansion, primarily driven by anthropogenic factors rather than climatic variability, led to a net increase in global reservoir capacity, particularly in Asia. While most reservoirs remained stable, distinct hotspots of construction and disappearance were observed. We highlight the impact of reservoir expansion on hydrological regimes. River flow regulation has intensified, particularly in headwater streams, a phenomenon largely underestimated in previous assessments. We suggest that continued reservoir construction may have complex implications for water availability and ecosystem health. Our findings provide critical insights into the global distribution and growth of human-regulated reservoirs, informing future water resource planning and policy-making.