Abstract The response of a long concrete gravity dam-reservoir system for a harmonic rigid base acceleration normal to the dam axis is investigated. The hydrodynamic pressure acting on the vertical wet surface of the dam is first evaluated in closed form as a function of the unknown deflections of the dam-reservoir interface and the ground acceleration. Employing finite element techniques, the motion of the dam is investigated. The hydrodynamic pressures enter the equation of motion of the dam as loadings in excess of the inertia load. In this approach, the general flexibility of the dam cross-section and the compressibility of water are taken into account. The viscosity of water and the effect of the surface waves are neglected. The coupling between the dam and the reservoir results in changes in the mass and stiffness properties of the dam which depend on the excitation frequency. For an excitation frequency greater than the fundamental frequency of the reservoir, the damping properties of the dam are also modified.