Thermal renaturation of rivers in the post-industrial age - An example of the Przemsza River basin (Poland)

Abstract

This article concerns temperature, which is one of the most important physical properties of surface waters. During the period of climate warming, tendencies of increasing river temperature have been repeatedly identified in the literature. This article discusses the lowering of the river temperature, a phenomenon rarely considered, which is occurring in a heavily industrialised area in the southern part of Poland (Upper Silesia Region), undergoing deep restructuring.The main aim of the study is to analyse and evaluate the unique Przemsza River basin, which differs from other river basins in terms of the thermal regime of its rivers and its tendencies for change. These changes are presented through the long period of 1961–2015, and additionally in two sub-periods, 1961–1994 and 1995–2015, differing with respect to the degree of the organisation of wastewater management. This was possible to demonstrate thanks to the high density of hydrological stations within a small area, which is rare, especially over such a long period. This work, therefore, fills the research gap on changes in the thermal regime of rivers caused by water management. Its results differ from those presented so far in the literature and expand the knowledge of this subject.In the years under consideration (1961–2015), the air temperature increased by 0.03 °C·per year−1 on average. It was found that the temperature of rivers (or their sections) decreased by as much as −0.8 °C·year−1. The greatest drops in temperature occurred in the winter season and amounted to a maximum of −0.12 °C·season−1. In the first part of the analysed period (1961–1994), unnaturally high river temperatures caused by water management factors were recorded. In turn, in the years 1995–2015 there was a decrease in temperature caused by various technical operations carried out in the catchments, including the construction of modern sewage treatment plants, as well as limitation of the introduction of water into rivers from closed hard coal mines. This phenomenon is referred to as thermal renaturation. Maintaining positive changes in the thermal regime requires, inter alia, a further reduction in the amount of pollutants discharged into rivers by water discharges from mine drainage.