The basic results of analysis of the data derived from the geophysical observations are as follows: the effect of strengthening grouting of the mass of the bed prior to its loading is established by a significant (in the probabilistic sense) increase in the spread velocities of elastic waves and by a reduction in their variation (the average velocity values of the longitudinal waves increased to 14% and their coefficient of variation in the near-contact layer on the basis of seismic-testing data decreased from 3.1 to 1% as a result of grouting alone; the value of the parameters (b, c) of the Weibull distribution diminished sharply); in the process of loading the bed with the dam and reservoir, the statistics of the distribution of geophysical indicators of the state of the mass (spread velocity of the waves, temperature, flow) vary significantly in each stage of the observations; the character of the variations with time assumes the form of oscillations corresponding to the loading process, the long-term effect of grouting is confirmed, firstly, by consistently significant excedence (right up to 1990) with respect to the initial (with the mass in the undisturbed state) average values of the velocity and the discrepancies in the other statistics of the velocity distributions (especially the coefficient of variation — 1.8 in 1990 versus 3.1% — and the shape parameter of the Weibull distribution C=17 versus C=11, see Fig. 1; and, secondly, by the reduction in flows immediately beyond the segment of the grout curtain and by their attenuation in the direction of the tailrace (Fig. 6); variations in the velocities of the waves (stress-strain state of the mass) occur synchronously, and, as a rule, with the same sign throughout the entire cross section of the bed; special cases, including repeating reductions in velocity, are obviously the result of the unloading of the compressive stresses, which preceed them and which can be established from the increase in velocities. This unloading is realized on local inclined elongated structural elements of the mass, which are recorded as velocity and temperature anomalies that can be identified with cracks (seepage channels); the structure of the deformed mass inherits its initial state, developing or smoothing over signs of its discreteness and homogeneity, for example, fully opening one and simultaneously closing neighboring cracks; and, the distribution of the elastic-deformation properties of the mass across its section in different loading stages has particular differences, retaining, however, the basic member of the section on the near-contact layer and underlying block (blocks). These values also vary according to changes in the shape and position of the zones of different mean values of the velocities and compression moduli with time (see Fig. 5). The experience gained with complex geophysical observations during loading of the bed confirmed the highly informative nature of the methods, and the possibility of providing for a representative (massive) and reliable investigation of the dynamics of the state of the mass and its structure over the entire depth assumed for the region of activity (for the case in question, apparently a greater assumed depth (to 50 m) of the observations, which had run counter to expectations). For the high sensitivity and low resolution threshold of the methods employed to measure the geophysical indicators of the medium, which are necessary and adequate for the investigations, however, the stresses and compression modulus are determined only by correlation in terms of the wave velocity without strict evaluation of accuracy, so that the values of these parameters indicated in the study should be assumed approximate. The weighted combination of geophysical and mechanical measurements is recommended for the planning of similar studies.