Monitoring the Transformation of an Aging Plain Reservoir: InSAR-Derived Deformation Associated with Dredging and Capacity Expansion at Suyahu Reservoir, China

Reservoir sedimentation reduces operational performance, while large-scale dredging and capacity expansion redistribute earth materials and alter engineering conditions at aging plain reservoirs. This study integrates Sentinel-1 time-series Interferometric Synthetic Aperture Radar (InSAR), multitemporal optical imagery, and ICESat-2 and Sentinel-3A altimetry to characterize the engineering transformation and associated deformation at Suyahu Reservoir, China. Optical imagery documented land-based excavation, cofferdam construction, and subsequent reinundation. Under comparable instantaneous water levels, the mapped water surface area increased from approximately 72.06 km2 in April 2020 to 87.45 km2 in April 2025, an increase consistent with changes in reservoir-bed topography and shoreline morphology. Satellite altimetry showed that the mean water level decreased from about 52.62 m before construction to 52.15 m during the main construction period, while the mean annual observed range narrowed from 1.33 to 0.68 m before increasing again from mid-2023 onward. InSAR revealed spatially heterogeneous negative line-of-sight velocities reaching approximately −40 mm/a along localized embankment sections and −50 mm/a along the monitored margins of the artificial islands. Lagged cross-correlation analysis identified no statistically significant correlations between water level variations and the monitored embankment displacement. The spatial correspondence between deformation and project-modified areas supports a plausible first-order interpretation based on local excavation, fill placement, and construction histories.