Temperature is one of the most important physical properties of substances occurring in the environment. An increase in air temperature (AT) observed in recent years is simultaneously the most characteristic and most important premise confirming climate warming. It causes an increase in the temperature of all elements of the environment, including river temperature (RT). This in turn usually results in unfavourable physiochemical, biochemical, and ecological changes in the aquatic environment. Additionally, it complicates rational and sustainable management of water resources.
The aim of the published work was to determine the magnitude and directions of changes in the thermal regime elements of nine rivers of different scale and directions of anthropogenic changes. Detailed studies were conducted based on unique series of daily water temperatures from sixty years (1961-2020), divided into two sub-periods: 1961-1992 and 1993-2020.
The analyses showed that thermal regimes of all studied rivers were different in both subperiods. Rivers were definitely warmer in the second subperiod, except for rivers under strong human pressure. The average increase in RT was greater in large rivers (Vistula +0.9oC, Odra and Bug +1.0oC) compared to medium rivers (Wda +0.7oC, Biebrza, Łyna and Rega +0.9oC). They resulted mainly from large-scale changes in climatic factors (global warming), and not local factors, including human impact.
The Przemsza River, which was heavily thermally polluted in the years 1961-1992, whose water temperature decreased by 1.2oC in the years 1993-2020 (due to renaturalisation) was the exception. At the same time, in the thermally polluted Ner River, a slight increase in RT (+0.4oC) was a result of long-term but multidirectional changes in water and sewage management.
The differences in the average daily temperatures in the river, between the sub-periods in large and medium rivers, were the greatest in spring and reached +3.5oC. In rivers under strong human pressure, they were clearly smaller than in the others: in Przemsza River, even negative (up to –2.2oC). Large changes were also recorded in the case of maximum daily RT, among which in the years 1993-2020 there were high increases reaching even 3oC. Przemsza, where large decreases in daily RT were recorded, reaching –3.7oC was also an exception here. In the thermally polluted Ner River, the maximum temperature in the river, in both sub-periods, was almost identical.
Source: Tomalski P., Pius B., Jokiel P., Marszelewski W., 2025. Changes in the thermal regime of rivers with different sizes and levels of human impact based on daily data (1961-2020). Case study from Poland. Geographia Polonica, vol. 98, no.1, 29-52