## 2 Materials and Methods
### Data Analysis
The final result was expressed on a scale from 0 to 100 points on a 5-point scale, i.e., 0-24.9 points--bad water quality (class V), 25.0-49.9 points--poor water quality (class IV), 50-74.9 points--moderate water quality (class III), 75.0-94.9 points--good water quality (class II), and 95.0-100 points--very good water quality (class I). A detailed description of the research methodology for the mentioned water quality indices (especially in terms of determination of sub-index values and methods of classification of results calculated for these indices) can be found in other articles [33; 45; 46; 47; 48; 49; 50; 51; 52; 54]. * This study also analyzed descriptive statistics and checked the obtained results regarding their statistical significance. For this purpose, a non-parametric Wilcoxon rank test was performed for data with a distribution not corresponding to the linear distribution, in which the null hypothesis is that two groups of variables (i.e., in this case, data upstream and downstream of individual hydropower plants) do not differ in the median [55]. The analyses were performed using the SPSS Statistics 26 software. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Index Name**} & **Data for the Index Calculation** & **Parameters Taken into Account** & **Final Index Value** \\ \cline{3-4}  & **Data for the Index Calculation** & **Parameters Taken into Account** & **Equation** & **Explanation of Symbols** \\ \hline \multirow{3}{*}{Oregon Water Quality Index (OWQI) [33; 45; 46; 54]} & \multirow{3}{*}{Median value} & DO, pH, BODs, NH\({}_{4}\) + NO\({}_{3}\), TP, temperature & \multirow{3}{*}{\(OWQI=11.1\)} & OVQI = the final index value, SI\({}_{1}\)—the sub-index value for each parameter, and n\({}_{-}\)the number of parameters considered in the calculations. \\ \cline{3-4}  & & & & \\ \hline \end{tabular}
\end{table} 
Table 6: Basic information on determining the water quality indexes described in the article (OWQI, OIP, DWQI, CPCB WQI, UWQI, and NSF WQI)—equations converted to a scale of index values from 0 to 100 points [33; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54]. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Parameter**} & **Compliance Rate of the Results (Percentile)** & **A Group of Fish** & **Requirement** \\ \hline \multirow{2}{*}{Temperature} & \multirow{2}{*}{98\%} & Salmonds & max 21.5 \({}^{\circ}\)C and \(\Delta\) 1.5 \({}^{\circ}\)C \\ \cline{3-4}  & & & Cypriinds & max 28.0 \({}^{\circ}\)C and \(\Delta\) 3.0 \({}^{\circ}\)C \\ \hline \multirow{2}{*}{DO} & \multirow{2}{*}{100\%} & Salmonds & min 7 mg/L \\ \cline{3-4}  & & Cypriinds & min 5 mg/L \\ \hline \multirow{2}{*}{pH} & \multirow{2}{*}{95\%} & Salmonds, cypriinds & 6–9 and max \(\Delta\) 0.5 \\ \cline{3-4}  & & & \\ \hline \multirow{2}{*}{BOD\({}_{5}\)} & \multirow{2}{*}{95\%} & Salmonds & max 3 mg/L \\ \cline{3-4}  & & & Cypriinds & max 6 mg/L \\ \hline \multirow{2}{*}{TP} & \multirow{2}{*}{95\%} & Salmonds & max 0.2 mg/L \\ \cline{3-4}  & & & Cypriinds & max 0.4 mg/L \\ \hline \multirow{2}{*}{NO\({}_{2}\)} & \multirow{2}{*}{95\%} & Salmonds & max 0.01 mg/L \\ \cline{3-4}  & & & Cypriinds & max 0.03 mg/L \\ \hline \multirow{2}{*}{NH\({}_{4}\)-N} & \multirow{2}{*}{95\%} & Salmonds, cypriinds & max 0.78 mg/L \\ \hline \end{tabular}
\end{table} 
Table 5: The requirements of the conditions for the life of salmonids and cypriinds according to Polish regulations [44].