Abstract: Reliable detection and monitoring of seepage hazards in embankment dams remain challenging. Conventional approaches, such as drilling, are costly and provide only point-scale information, whereas geophysical methods often rely on indirect proxies that limit intuitive interpretation of seepage processes. To enable efficient and intuitive characterization of seepage hazards, this study proposed a spatiotemporal imaging approach for water content based on high-density electrical resistivity tomography (ERT). In the area of Fangli Reservoir in Yantai City, water content was selected as a direct indicator of seepage hazards. Correlations between water content and geoelectrical parameters of soil and rock-core samples were analyzed to establish a quantitative prediction model. A mixed Lp-norm-constrained inversion method, a 3D geoelectrical reconstruction workflow, and temporal constraints were further introduced to improve the inversion of monitoring data. Two high-density ERT lines were deployed on the downstream slope, and monitoring surveys were conducted during and after the flood season to obtain spatiotemporal images of resistivity and water content. Fiber-optic and hydrogeological monitoring data were used for validation. The imaging results reveal a shallow sandy loam layer with high water content on the downstream slope and delineate its three-dimensional distribution characteristics. Time-lapse imaging indicates that water content increases in some areas during the monitoring period. Displacement, water-level, and water-temperature data suggest no significant structural damage to the embankment. High-water-content anomalies may be associated with insufficient soil compaction, precipitation, and potential seepage. Early warning and strengthened follow-up investigation and monitoring are therefore recommended. The results demonstrate that water-content-based spatiotemporal imaging is effective for seepage hazard detection and monitoring in reservoir embankments.