Abstract: Existing methods for detecting seepage hazards in embankment dams have notable limitations. Drilling is costly, inefficient, and provides only localized information, whereas geophysical methods usually rely on indirect interpretation of subsurface parameters and thus cannot intuitively reveal seepage hazards. To enable efficient and intuitive characterization of seepage hazards and monitoring of their evolution, this study proposes a spatiotemporal imaging approach for water content based on high-density electrical resistivity tomography (ERT). Taking Fangli Reservoir in Yantai as the study area, 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 revealed a shallow sandy loam layer with high water content on the downstream slope and its three-dimensional distribution characteristics. Time-lapse imaging showed that water content increased in some areas during the monitoring period. Displacement, water-level, and water-temperature data indicated no significant structural damage to the embankment. High-water-content anomalies may be associated with insufficient soil compaction, precipitation, or 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.