Abstract. Floodplain lakes (FPLs) constitute a very important element of river valleys, both in terms of ecology and hydrology. Dynamic physicochemical, morphometric and biological changes of lake waters are determined by the variability of the functional periods of lakes: limnophases, potamophases and inundations. This paper presents factors that shape long-term dynamics of the frequencies and durations of potamophases and limnophases in 20 selected FPLs. The study area included the left fraction of the Bug River valley located at the European Union's eastern border stretched along countries like Poland, Ukraine, and Belarus. The analysis covered the water years 1952 to 2013. Assigning the value of Limnological Effective Rise (LER) was essential for determining the functional periods for each of the study lakes. The dynamics of the phenomenon was analysed using volatility indicators, while factors determining functional periods were distinguished using Principal Component Analysis (PCA). Results showed that short (0–8 days) and medium-length limnophases were observed most frequently during the study period. In the case of potamophases they most often lasted from 8 to 30 days, continuously. Double-mass curves showed four periods of increasing significance of one of the functional phases: 1952–1962 (limnophases), 1963–1982 (potamophases), 1983–1997 (limnophases) and 1998–2013 (potamophases). A variability that was observed in each floodplain lake under study resulted from two main factors: water input and lake basin morphometry. The major role in FPLs' input was played by potamic supply (inflow of water from the parent river), which was a derivative of Bug River water stages and discharge. Atmospheric precipitation played a smaller role. However, the role of local precipitation was marginal in relation to precipitation in the upper part of the Bug River catchment. Spatial variability of the frequencies and durations of potamophases and limnophases was also associated with the topography of the floodplain lake catchments. Hydrological connection to the river favoured (confluent lakes) or limited (contrafluent and contrafluent–confluent lakes) the frequencies of potamophases in the study period of 62 years.