Data regarding 25 lakes (9 Belarusian and 16 Polish) provided the basis for spectral analysis of water level fluctuations. The obtained output data concerned a 55-year observation sequence covering the years 1956-2010. The selection of lakes was determined by two factors, i.e. data continuity and inconsiderable anthropogenic impact. The lakes were divided into three groups depending on the course of the water level spectrum. The first group included the highest number of lakes (14) which were distinguished by smooth curves with no significant peaks (e.g. Lake Wygonoszczańskie). The second group shows a four-year variability (e.g. Lake Łukomskie). The third group is distinguished by an even peak of three-year water fluctuation (e.g. Lake Łebsko). The location of the lakes categorised into those three groups suggests a certain regionalisation. The first group is represented by lakes of central and northern Belarus, and eastern Poland. The second group includes lakes of eastern Belarus, and the third group - lakes of central and northern Poland. This suggests that alongside local factors, the location of the distinguished groups of lakes may also be determined by regional factors. Presumably, continentalism of climate increasing in an eastwards direction is one of the key factors responsible for the detected regional diversity of lake water level fluctuations.

The article presents the analysis of the formation of floods in Polesie on the r. Pripyat during the period of instrumental observations. Polesie is a unique physiographic region, located on the territory of four countries: Belarus, Poland, Russia and Ukraine. The total area is about 130 thousand km2. The Pripyat, the main river of Polesie, is an average river according to the European scale. The length of the r. Pripyat is 761km; the catchment area is 173.7 thousand km2. The article describes the most outstanding floods. The basic measures to reduce the negative effects of floods and priorities for floods research in Polesie are outlined

The research results of runoff changes in the River Viliya at 3 stations (Steshitsy Village, Vileyka Town and Mihalishki village) during the period 1946–2014 for the average annual, maximum, minimum summer-autumn and winter runoff are presented. It has been concluded that heterogeneity in the time series of the river runoff is caused by natural-climatic and anthropogenic factors. At Mihalishki Village the average annual runoff is about 59.7 m³ s–1, the maximum 1570 m³ s–1, minimum summer–autumn is 22.0 m³ s–1, the minimum winter runoff is 17.3 m³ s–1, and the environmental runoff is 21.1 m³ s–1. A forecast of runoff changes for the River Viliya, depending on forecasted climate change using the “Atlas of Global and Regional Climate Projections” was made on the basis of four scenarios RCP8.5, RCP6.0, RCP4.5, RCP2.6. The results of research indicate that significant changes in runoff will not occur as the forecasted climatic parameters did not change significantly. A forecasted decrease in spring runoff was investigated, thus reducing the minimum runoff is not essential. In the event of possible low water periods the Vileyka reservoir resources, involving the Olkhovskoye and the Snigyanskoye water reservoirs, can be used for compensation measures, which may be considered as the most reliable backup source of industrial water supply for the Belarusian Nuclear Power Plant.

The paper presents long-term changes in water temperature in two rivers, Oder and Neman, with catchments showing different climatic conditions (with dominance of marine climate in the case of the Oder and continental climate in the case of the Neman River). A statistically significant increase in mean annual water temperature was recorded for four observation stations, ranging from 0.17 to 0.39 °C dec⁻¹. At the seasonal scale, for the winter half-year, water temperature increase varied from 0.17 to 0.26 °C dec⁻¹, and for the summer half-year from 0.17 to 0.50 °C dec⁻¹. In three cases (Odra-Brzeg, Odra-Słubice, Niemen-Grodno), the recorded changes referred to the scale of changes in air temperature. For the fourth station on Neman (Smalininkai), an increase in water temperature in the river was considerably slower than air temperature increase. It should be associated with the substantial role of local conditions (non-climatic) affecting the thermal regime in that profile. Short-term forecast of changes in water temperature showed its further successive increase, a situation unfavorable for the functioning of these ecosystems.