MULTIMODAL CHARACTERISATION OF ERELU RESERVOIR BED, OYO STATE NIGERIA.

This study presents a comprehensive multimodal assessment of the Erelu Reservoir by integrating bathymetric surveying, side-scan sonar (SSS), GNSS-based topographic data, and multi-temporal satellite imagery to examine its morphological evolution and sedimentation processes. Depth measurements taken across multiple survey periods (February 2022 to August 2025) ranged from 0.45 m to 5.70 m, with an average of approximately 3.1 m. Over the same period, the estimated storage volume declined from about 2.69 × 10⁶ m³ to 2.16 × 10⁶ m³, while the navigable surface area decreased from roughly 95 ha to 71 ha, indicating gradual infilling and capacity loss.Side-scan sonar imagery identified three main acoustic facies—medium-sand channels, soft-mud deposits, and coarse backscatter patches—and revealed human-induced features such as a submerged canoe and a buried pipeline that influence local sediment trapping and flow dynamics. Land Use and Land Cover (LULC) analysis showed a significant shift from mainly agricultural and vegetated areas in 1984 to an increasingly urbanized landscape by 2024, a change that aligns with higher sediment inflow rates.Overall, the integrated bathymetric, sonar, and LULC findings demonstrate a strong connection between terrain alteration, surface runoff, and sediment deposition contributing to the reservoir’s degradation. These results emphasize the importance of continuous hydrographic and sonar monitoring, alongside effective catchment management strategies such as reforestation, vegetative buffer establishment, contour farming, and selective dredging of sediment-heavy zones. Future research should incorporate sediment-core sampling and hydrodynamic modeling to improve understanding of sediment sources and estimate the reservoir’s remaining lifespan, while also addressing uncertainties linked to single-beam data density and interpolation accuracy.