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A small earth‐cored dam, the water balance of which indicated water losses, was chosen to test the applicability of the underwater self‐potential (SP) method. The data were recorded using a measurement system composed of an electrode dragged over the upstream face of the dam and attached to a well‐logging instrument. The measured electrical potential was referred to a fixed point inside the reservoir and near to the pool water level. The obtained SP data were split into regional and local components. The regional component was generated using an existing correlation between a vertical SP water gradient and the water depth of the upstream slope shape of the dam. The residual SP field was obtained by subtraction of the regional component from the recorded data. The residual SP field showed several anomalies, one of which coincided with the axis of the outlet conduit. Depth estimation gave a value equal to the position of the tube. Some of the anomalies suggested the presence of suffusion processes that had developed along the upstream slope, similar to sink‐hole dissolution. The SP anomalies and the conductive bodies interpreted from the direct current measurements which were carried out on the crest of the dam, showed good spatial correlation; this indicated the possibility of hydraulic connections through preferential flow paths between the upstream face and the body of the dam. In quantitatively interpreting the SP anomalies, their relative positions on the upstream face must be taken into account, due to possible electrical‐potential distortions at the rock‐fill/water and water/ air limits.