| Summary: | Ureje Dam, Ado-Ekiti has witnessed drastic reduction in the water storage capacity of its reservoir. It became imperative to determine the possible cause(s) of the reduction in storage capacity. Geophysical investigation involving the vertical electrical sounding technique of the electrical resistivity method was conducted in the upstream part of the dam. Five lithologic units that include the mud/suspended materials, such as sandy clay, clay, weathered/fractured bedrock and fresh bedrock, were delineated. The respective resistivity and thickness range of the units are 2–19 ohm-m; 147–206 ohm-m, 2–38 ohm-m; 47–236 ohm-m and 455–1516 ohm-m and 0.4–1.9 m; 0.5–2.5 m; 1.0–12.2 m; 7.3–16.4 m and ∞. The thickness of suspended materials, resistivity/thickness of weathered layer and the presence of near-surface impervious layer were used as the main indices for the spatial demarcation of the dam axis in terms of vulnerability to loss of impounded water. Using the cumulative response of the indices, the study concluded that the eastern to southeastern parts of the dam axis showed the highest indications of vulnerability to loss of impounded water.
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