This study presents a bathymetric survey quantifying sedimentation levels in Kiri Dam Reservoir, Nigeria, covering the operational period of 43 years (1982-2025). The report compares the original survey data with a geodetically controlled hydrographic resurvey conducted in 2025. The employed methodologies strictly adhered to the standards outlined in IHO S-44 (2020), including Trimble RTK-GNSS control, Acoustic Doppler Current Profiler (ADCP) depth soundings, crossline calibration, uncertainty propagation, triangulated irregular network (TIN) and raster surface generation, as well as hypsographic volume integration. At NTWL (170.5 meters above mean sea level), live storage has diminished from 615 million cubic meters to 344.15 million cubic meters, representing a reduction of 44.04%. Additionally, the reservoir surface area has decreased from 106.36 to 67.02 square kilometres, a decline of 37%. These sedimentation losses are predominantly concentrated within the 161– 167 metre benches, as confirmed by Particle Size Distribution (PSD) analyses of silty clay and geochemical fingerprinting, indicating the presence of organic-rich fines. Regression analyses demonstrate strong linear relationships between elevation and capacity, with R² values of 0.996 for 1982 and 0.992 for 2025. The findings indicate a reduction in irrigation reliability and flood buffering capacity, underscoring the necessity for targeted sediment management strategies and periodic resurveying to ensure sustainable reservoir operation.