Water temperature is one of the basic parameters of rivers and lakes. Its commonly observed increase contributes to the transformation of those ecosystems, and covers a broad range (water mixing, water quality, biological conditions, etc.). In the case of rivers, their thermal regime can be modified by local conditions (tree stands, water bodies, etc.). The paper presents the functioning of the river-lake system in North Poland (Kocunia River – Lake Sławianowskie) in terms of the effect of the lake on temperature conditions in the river. The assessment of dependencies between water temperature measures at the three aforementioned stations employed the linear regression model. Based on daily water temperatures in the period 2012-2017, it was determined that water temperature in the river below the lake was higher than that at the measurement site located above the lake by an average of 1.1°C. The greatest differences were recorded in summer-autumn months when average monthly water temperatures in the river above and below the lake differed by as much as 3.9°C. The aforementioned situation should be considered unfavourable due to the fact of magnification of global factors causing an increase in water temperature in rivers by local factors (presence of a lake). Information included in the paper can provide the basis for future reference for persons or state authorities in charge of measures aimed at the reduction of effects of climate change.