The determination and monitoring of deformations that may occur in the benches of dump sites of open-pit mines are important for both security and continuity of mine production. The Global Navigation Satellite System (GNSS) is the most preferred method for deformation monitoring at dump sites. While the amount of deformation (3D displacements) can be determined at the points monitored by the GNSS method, interpretations on the cause and/or areal size of the deformation may remain limited. The main factors or triggering causes of slope failures at dump sites are base topography, geometric dimensions (width, height, and slope) of the formed benches, stiffness ratio of the dumped material, and load increases caused by new dumps in the lower benches. In this study, UAV photogrammetry and GNSS were used together, and the deformations at the adjacent dump site of three different open-pit marble mines located in Eliktekke region of Amasya province in Turkey were monitored. The GNSS method determined the points where displacements occurred, and the UAV photogrammetry searched for the causes of mobility at the point and its areal and volumetric sizes. The UAV photogrammetric results revealed that the topographic slope and overall slope inclination triggered the movement of the zones. Field measurements and evaluations made for this dump site showed that the current geometry should be kept where the overall slope inclination reaches 14 degrees and not to be increased further. The results support that the GNSS method assisted with UAV photogrammetry is a rapid and economical method to determine both the borders and volumetric sizes deformations as their causing parameters. Through the proposed methodology, it will be possible to effectively determine the main factors causing slope failures in similar areas and to take the necessary measures on time.