ABSTRACT: The contribution reports the experience related to groundwater monitoring at two selected Slovak dams before backwater and primarily the experience of change analyses in flow modes during long term both dams operation. The development of ground and seepage water parameters by both dam bodies and dam subsoil indicates that the major changes occur after the first filling of reservoir. However at the same time since the first backwater there can be monitored a long term process of the groundwater level development trend primarily and in minority also the seepage amount since the first backwater in reservoir. The development of flow mode changes until now still does not indicate jeopardy of filtration stability of dam bodies and dam bedrocks.