Ecological Systems and Devices Annotation << Back Harmonic Analysis of River Water Quality Rozental O.M., Krokhin G.B An attempt was made to fi nd hidden patterns in the series of substances that form river water quality. Using water quality data from the Iset River as an example, some time series that seem chaotic at fi rst glance were analysed. The analysis allowed to identify the frequencies and associated harmonics that contribute the most to the process. This in turn can help predict the periodicity of the occurrence of increased concentrations and adapt the water usage to the current conditions. The nature of the detected phenomenon is not fully understood; it may be related to hydrological factors such as "long memory" and self-organisation of systems, as well as to anthropogenic processes. In any case, it is of scientifi c and practical interest in planning water management decisions to clarify the ordering mechanisms of the seemingly chaotic time series in question. научный и практический интерес при планировании водохозяйственных решений. Keywords: harmonic analysis, approximation reliability, water quality, thermodynamic equilibrium, pollutant concentration. DOI: 10.25791/esip.11.2023.1408 Pp. 03-9.