We applied refraction seismic and electrical (induced polarization and self-potential) methods to study an internal structure of an earth dam. The dam 1400 m long produces a strong hydraulic head difference (of about 40 m). It forms a large water storage basin and is a part of hydroelectric power plant. Combined analysis of seismic and electrical data allowed us to characterize the piesometric surface within the dam and to obtain the bedrock depth. Based on induced polarization and resistivity data we assessed the transmissibility of an aquifer present within the dam. Self-Potential data show large values of electrical potential of electrokinetic origin because of very low electrical conductivity of surface and groundwater. The Self-Potential distribution was first analyzed qualitatively, which allowed determining two areas: the first area is characterized by a uniform groundwater flow, and the second area shows a complicated strongly three-dimensional flow. Then the groundwater flow model was obtained on the basis of ModFlow software. The Self-Potential was also calculated based on hydraulic head and electrical resistivity distributions. By trial and errors the initial model was corrected in order to obtain an agreement between the measured and modeled Self-Potential signals.