This case study aims to identify the cubic capacity and geometry of the geological body of silt-organic sediments in the environment of a former gravel pit situated in a drainless depression of the alluvium of the Čierna voda River. It is located in the well-known geotourism locality of Slnečné Jazera in Senec, in the south-west of Slovakia. In order to identify the body, the method of electrical resistivity tomography was combined with the use of a sonar. The research shows that the methodological approach is appropriate for a number of activities that are subjects of engineering-geological investigations. The cubic capacity and geometry of specific aqueous engineering-geological environments must be determined in connection with the need for the management, control, quantification, and extraction of selected sedimentary bodies. In addition, the management of sustainable development of reservoirs, sedimentation basins, industrial ponds, settling pits, and natural pools for recreation (the subject of the case study) is important to control the limit amounts of sediments in such environments. The results of the research study may be applied in analogous engineering-geological conditions. The drainless depression Slnečné Jazera contained a body of silt-organic sediments amounting to 23 000 m3 (41 Olympic-size pools of 25 x 12.5 x 1.8 m). The maximum thickness of the bottom sediments was about 6.3 m on the alluvium with an articulated morphology due to the submerged digging of gravel. The proposed methodology improved the control of extraction of the sedimentary body and optimized the engineering-geological conditions in the geotourism locality in question.