MULTIMODAL CHARACTERISATION OF ERELU RESERVOIR BED, OYO STATE NIGERIA.

ABSTRACT This study presents a detailed multimodal evaluation of the Erelu Reservoir, employing bathymetric surveying, side scan sonar (SSS), GNSS-referenced topographic measurements, and multi-temporal satellite imagery to assess its morphological changes and sedimentation dynamics. Depth measurements obtained across survey periods (February 2022 to August 2025) ranged from 0.60 to 5.70 m, with an average depth of approximately 3.01 m, while estimated storage volumes varied from about 2.16 × 10⁶ m³ to 2.69 × 10⁶ m³. The navigable water surface area also declined from roughly 95 ha to about 71 ha, indicating continual infilling and loss of storage capacity. Side scan sonar imagery identified three main acoustic facies: medium sand channels, soft mud deposits, and coarse backscatter zones, and further detected anthropogenic elements such as a canoe wreck and a buried pipeline that affect sediment trapping and flow distribution. Land-use and land-cover (LULC) analysis demonstrated a shift from largely agricultural and vegetated surroundings in 1984 to a predominantly urbanised setting by 2024, corresponding with increased sediment delivery. The combined bathymetric, sonar, and LULC results show a clear geomorphodynamic relationship contributing to reservoir deterioration. These findings underscore the importance of continuous hydrographic and sonar assessment, the application of catchment erosion control measures (reforestation, vegetative buffers, and contour farming), and focused dredging of depositional regions. Future studies should incorporate sediment core analyses and hydrodynamic modelling to improve source identification and estimate the remaining lifespan of the reservoir, acknowledging existing limitations related to single-beam sampling density and interpolation techniques.