ABSTRACT: Water-work construction on a surface stream represents a technical interference with the environment, which is followed by impacts on the surrounding area. One of them is the interference with the natural groundwater regime. Methods of impact investigation are presented in the paper, illustrated on the case study of the water reservoir Kráľová. Its building and following utilisation influenced groundwater regime on both sides of a river plain of the river Váh. As a consequence of building of a right-side underground sealing wall and side dams, as well as of water reservoir filling in years 1978-1985, change in groundwater flow direction and groundwater level characteristics occurred, mainly in the right-side river plain. Changes were documented by a relatively quick increase of groundwater levels and their stabilising on a new, higher level. As a consequence, cellar rooms in the surrounding villages started to stay under water. Utilisation of mathematical-statistical methods in the process of grou nd water level changes analysis is presented, inc1uding analysis of research results for monitoring wells located on both sides of the stream in the surrounding of the water reservoir.