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The paper presents the results of a study into the stability of water-level regimes in Polish lakes. Stability is defined as a degree of regularity with which a certain phase of the hydrological regime occurs (high and low water levels). The hydrological regime is described with the use of the following 6 variables – first, second and third maximum; first, second and third minimum. Stability of the hydrological regime is determined by the stability coefficient (WS). The analysis relies on data concerning average monthly values of water levels for 34 lakes located in Polish lake regions. There are characteristic spatial differences in the stability of maximum and minimum water levels which are usually caused by environmental factors and human activity. It was observed that the most stable dates were those of the 1st maximum and the 1st minimum. In the case of the lakes in the Pomeranian, Wielkopolsko-Kujawskie and Mazurian lake districts, the occurrence date of the 1st maximum is highly stable and stable, while the date of the 1st minimum is the most stable in the lakes of the Drawsko, Wielkopolsko-Kujawskie and Mazurian lake districts. 7 305 315 22 Badania Fizjograficzne

The study area covers watersheds in the upper and middle parts of the Oder basin down to Słubice. This region is interesting in terms of its diverse climatic, hydrological, geomorphological and environmental conditions. The study contains hydrometric data from 94 gauging stations located on 53 rivers from 1971 to 2010. Calculations were made of the outflow for the entire period in question as well as for years with high and low values of the winter NAODJFM index. On the basis of a test of the significance of differences in the outflow between the average period and the years in positive and negative phases of the NAODJFM, the effects of the changes in the intensity of the North Atlantic Oscillation on the flow of the rivers in the basin of the upper and middle Oder were examined. In a negative NAODJFM phase, in 90 catchments the outflow was above the average. In a positive NAODJFM phase, the annual outflows on most rivers were below the average. 7 317 329 23 Badania Fizjograficzne

This article seeks to determine the regime of water stages of two lakes, Lednica and Powidzkie, located in the Wielkopolskie Lake District. In the research, use was made of their daily water-level figures from 1984–2012. In order to establish the regime features of the lakes, the method hitherto applied in river regime studies was employed: an analysis of the temporal structure and type of lake water stages and their changes in an average annual cycle. As the research has shown, Lake Lednica has a 5-period regime structure, while Lake Powidzkie has no characteristic cycle of change in its water levels, a consequence of which is the 2-period structure of its regime. Because of the similarity of the location of the lakes and the natural conditions of their operation, the causes of their different regimes should be sought in their individual features and in human activity. 6 131 142 10 Badania Fizjograficzne

This article examines the dependence between monthly and seasonal NAO indices and monthly flows of rivers in the Wisła basin down to Jagodniki. In the analysis, use was made of monthly flow data from 55 gauging stations from 1971–2010. On the basis of 156 coefficients of correlation among the variables analysed, two variants of the clustering of the gauging stations, using Ward’s method, were employed. In both, six river clusters were distinguished. In a majority of cases, the analyses showed there to be the strongest negative correlation between winter NAO indices and May to September flows of the Wisła’s left-bank tributaries. The NAO impact on the flow of the Wisła’s Carpathian tributaries is weaker, though still observable. 6 195 203 15 Badania Fizjograficzne