The article reviews the technology of construction and functioning of watertight elements of hydrotechnical embankments. A case study of two reservoirs located in the middle course of the Nysa Kłodzka River is described: (1) Otmuchów reservoir functioning since 1933, (2) Nysa reservoir functioning since 1971. The effectiveness of the applied impermeable screens, i.e. a clay screen of the Otmuchów reservoir and a concrete screen of the Nysa reservoir, was assessed and compared. Data from piezometers located at the height of the dam crest along its entire profile were analysed for measurements taken in the years 2020-2022. Indicators such as the distribution and depth of the water level in the dam body and changes in pressure in piezometers under the influence of water level fluctuations in the reservoir were considered. In addition, the scale and frequency of repair and maintenance activities carried out on both facilities were analysed, especially in aspects of the dam body sealing and the need to repair the upstream slope structures, which are directly exposed to filtration and wave action. It was found that, despite the long period of operation of both facilities, they maintain proper water tightness, and the depth of the water level in the embankments reacts only slightly to fluctuations in the amount (height) of stored water.