Issues, related to the clarification of hydrodynamic pressure on arch and gravity dams, were the topic of all last congresses on seismic resistance, because they are directly connected with reliability control of these important facilities. Impact of water pressure on the dam in case of earthquake depends on a number of factors, including vibratory energy absorption by the bottom of the reservoir that plays a significant role. However the determination of the absorption parameter involves considerable difficulties, because it is influenced by properties of a bottom material and alluviums, silt, debris and so on. It makes this characteristic substantially uncertain. Stationary and non-stationary plane problems of hydromechanics for a semi-infinite fluid layer of finite depth are considered. Hydrodynamic pressure on the vertical upstream face of the dam under the horizontal seismic load with energy absorption on the reservoir bottom and a layer of sediment is determined. Uncertain factors are the amplitude of the impact and the absorption parameter. Point estimation (based on probability theory) and interval estimation (using theory of possibilities) of the spread of hydrodynamic pressure on dam under acoustic resonance and non-resonance conditions are given.