## 3 Results and Discussion
### Multivariate Analysis
Meanwhile, the Piaskowa, Ukleja, Lubieszowa, Molstowa, and Brzezenicka-Wegorza rivers exhibited similar physicochemical profiles and clustered closely. The Gardominka and Rekowa rivers formed another subgroup, with distinctively low average TSS and DO values (Table 6). These rivers are heavily influenced by beaver activity, especially the Gardominka, where numerous dams create extensive backwater areas. The slowed flow promotes sedimentation and enhances aerobic decomposition of organic matter, both of which contribute to oxygen depletion. \begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline  & **T** & **pH** & **TSSs** & **DO** & **BOD5** & **NH4\({}^{*}\)-N** & **NO2\({}^{-}\)-N** & **TP** & **EC** & **Alc.** & **TH** \\ \hline pH & \(\mathbf{-0.17}\) * & & & & & & & & & & \\ TSS & \(-\)0.01 & \(-\)0.05 & & & & & & & & & \\ DO & \(\mathbf{-0.58}\) * & \(\mathbf{0.41}\) * & \(\mathbf{0.17}\) * & & & & & & & & \\ BOD5 & \(\mathbf{-0.18}\) * & \(0.14\) & \(0.03\) & \(\mathbf{0.45}\) * & & & & & & & \\ NH4\({}^{+}\)-N & \(0.03\) & \(-\)0.04 & \(\mathbf{0.17}\) * & \(-\)0.04 & \(-\)0.03 & & & & & & \\ NO2\({}^{-}\)-N & \(0.15\) & \(0.13\) & \(0.05\) & \(-\)0.05 & \(\mathbf{0.51}\) * & & & & & & \\ TP & \(\mathbf{0.28}\) * & \(-\)0.02 & \(\mathbf{0.19}\) * & \(-\)0.07 & \(-\)0.13 & \(\mathbf{0.43}\) * & \(\mathbf{0.40}\) * & & & & \\ EC & \(0.04\) & \(-\)0.04 & \(0.03\) & \(0.14\) & \(-\)0.02 & \(\mathbf{0.29}\) * & \(\mathbf{0.38}\) * & \(\mathbf{0.47}\) * & & & \\ Alc. & \(0.15\) & \(0.29\) & \(\mathbf{-0.22}\) * & \(0.09\) & \(-\)0.01 & \(-\)0.00 & \(0.07\) & \(\mathbf{0.25}\) * & \(\mathbf{0.41}\) * & & \\ TH & \(\mathbf{-0.29}\) * & \(0.26\) & \(-\)0.10 & \(\mathbf{0.19}\) * & \(0.01\) & \(0.05\) & \(0.15\) & \(-\)0.04 & \(\mathbf{0.27}\) * & \(\mathbf{0.36}\) * & \\ Cl\({}^{-}\) & \(0.02\) & \(0.02\) & \(-\)0.08 & -0.03 & \(0.02\) & \(\mathbf{0.34}\) * & \(\mathbf{0.38}\) * & \(\mathbf{0.51}\) * & \(\mathbf{0.54}\) * & \(\mathbf{0.22}\) * & \(\mathbf{0.25}\) * \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Pearson correlation coefficients (r) for relationships among water quality parameters in the studied rivers. Supplementary PCA conducted separately for each river (Supplementary Figure S1 and Table S1) revealed that the first two components explained between 43.6% and 64.0% of the total variance, with the lowest and highest values observed in the Piaskowa and Gardominka rivers, respectively. Including the third principal component (PC3; see Supplementary Table S1) increased the cumulative variance from 59.60% (Piaskowa) to 73.68% (Gardominka). These values can be considered low, consistent with the findings of [PERSON] et al. [58], who reported a cumulative variance of hydrochemical parameters below 68%. They attributed this to incomplete hydrochemical data caused by water cut-off during the dry season. Similarly low cumulative variance values (>70%) have been reported in other studies, including those on groundwater systems with highly variable hydrological and geological regimes [59], stream waters [23], lake waters [60], and coastal waters [61], where pollution sources are diverse and include agricultural non-point source pollution, domestic sewage, industrial wastewater, and atmospheric deposition. In our study, we observed that the influence of individual water quality indicators on the extracted principal components (PCs) varied across the rivers. At the current stage--particularly due to the lack of detailed inventories of pollution sources within the catchments--it was not possible to combine PCA with factor analysis (FA), as was performed by, for example, [PERSON] et al. (2025) [23]. The application of PCA/FA would allow for more accurate identification of links between specific water quality indicators and actual pollution sources in each river. Nevertheless, it is worth noting that in our analysis, PC1 most frequently showed medium to strong correlations (above 0.6) with quality indicators, particularly in the Brzeznicka-Wegorza, Gardominka, Molstowa, and Sapolna rivers.