RESEARCH ARTICLE What factors affect the suspended sediment concentrations in rivers? A study of the upper Warta River (Central Poland) K. Skolasińska 1 | B. Nowak 2 1 Institute of Geology, Adam Mickiewicz University in Poznań, Poznań, Poland 2 Institute of Meteorology and Water Management – National Research Institute, Warsaw, Poland Correspondence Katarzyna Skolasińska, Institute of Geology, Adam Mickiewicz University in Poznań, Bogumiła Krygowskiego 12, 61‐680 Poznań, Poland. Email: katskol@amu.edu.pl Abstract The suspended sediment concentration (SSC) of the Warta River was analysed using data collected at the Sieradz gauging station (Central Poland) during the period of 1961–1980. The aim of the study was to characterize the trend in the suspended sediment transported over this multi‐annual period and search for possible correlations between the suspended sediment values and the discharge and thermal seasonality factors. This study also investigated whether the SSC is affected by anthro- pogenic factors. The SSC in the river water increased over the analysed 20‐year period. It was caused mainly by the training works in the Warta valley upstream of Sieradz and the opening of the Bełchatów lignite mine. No direct relationship between the SSC and fluctuations in the discharge was noted. A connection existed between the SSC and ice phenomena on the river. The river ice breakup was often coincident with increases in SSC; however, the SSC increased rapidly during the freezing of the river. There was also a positive correlation between the SSC and the temperature of the water during the summer half‐year, which resulted from the growth of phytoplankton. This process was supported by the general warming trend observed in the river water and by an increase in the quantity of nutrients noted since the early 1970s. It is worth emphasizing that the relationships established between the SSC and the studied factors are not always unequivocal and repetitive. It follows that, as a rule, the SSC is influenced by more factors than are actually considered in this study. 1 | INTRODUCTION Suspended sediment concentration (SSC), which is defined as the total value of both mineral and organic material carried in suspension by a river (Fryirs & Brierley, 2013), is dependent on many factors. The most important factors include the geological framework, topography of the drainage basin, and climatic conditions (Walling & Moorehead, 1989). The number of tributaries and the land use in the catchment basin, as well as the presence of river regulation works, hydrotechnical facilities, and other human activities, may play significant roles locally (e.g., Łajczak, 2003; Nelson & Booth, 2002; Siakeu, Oguchi, Aoki, Esaki, & Jarvie, 2004; Warrick & Rubin, 2007). Because of these factors, the transport of suspended sediment is highly non‐linear in time and space (Jarocki, 1957; Olive & Rieger, 1992). In a review paper, Vercruysse, Grabowski, and Rickson (2017) wrote, “despite decades of research, the spatial and temporal dimensions of the factors and process interac- tions underlying suspended sediment transport in rivers have not been fully captured and understood.” Analyses of the SSC in various rivers around the world showed that the SSC can vary considerably, both during individual hydrological years and during multi‐year periods (Walling & Fang, 2003). Analyses of the SSC in several major rivers in Poland (initiated in the beginning of the 1950s) also indicate that the SSC ranges quite widely (Brański & Banasik, 1996; Mikulski, 1961). The highest SSC has been reported for rivers flowing through mining areas, where the SSC on some days reaches values up to 1,000–2,000 g/m 3 as a result of mine water contamination. Moreover, a high SSC is reached in highland (200–300 g/m 3 ) and upland rivers (values close to 100 g/m 3 ). Significantly, lower SSCs are reported for lowland rivers (20–40 g/m 3 ), and the concentration decreases over the course of such rivers. The SSC during different months of a given year depend on several factors (frequency and intensity of precipitation, drought periods, frost periods, etc.). Therefore, the monthly SSC distribu- tion varies significantly during different years. Olive and Rieger (1992) noted that in order to draw appropriate conclusions on changes in the SSC, large datasets comprising at least 10‐year sequences are needed. However, there are not enough such data, and most studies have been based on datasets that are poor and incomplete. Thus, determination of the relationship between the SSC and the factors that control it is a difficult task. The SSC data used in this study come from the 1961–1980 period, but it should be Received: 25 January 2017 Revised: 30 October 2017 Accepted: 3 November 2017 DOI: 10.1002/rra.3234 River Res Applic. 2017;1–12. Copyright © 2017 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/rra 1