## 3 Results
### The Hydrochemical Characteristics of Water in the Kwacza River before and after Restoration  
The results of the analysis of discrete samples collected before and after restoration from 10 sampling sites between the up-stream (K1) and down-stream (K10) segments of the Kwacza River, and from the main Slupia channel as the reference site, are shown in Table 1. The values of most water quality parameters were higher in the Kwacza River than in the Slupia River. Nutrient concentrations (PO4\({}^{3-}\)-P, NO3\({}^{-}\)-N, and NH4\({}^{+}\)-N) were elevated along the studied section of the river, in particular before restoration. During this period, a decrease in pH and nitrogen compounds was observed between sites K1 and K10, and an increase was noted in DO and Cl\({}^{-}\).  
A comparison of selected physical and chemical parameters between K1 and K10 before and after restoration revealed considerable differences (Figure 3). The values of most parameters clearly decreased after restoration, and the greatest decrease was noted in pH, DO, NO3\({}^{-}\)-N, and NH4\({}^{+}\)-N (\(U\) test, \(p<0.0001\)). The mean values of EC, Cl\({}^{-}\) (\(p<0.001\)), and Chl-a (\(p<0.01\)) decreased significantly, whereas the drop in PO4\({}^{3-}\)-P concentration was on the brink of statistical significance (Figure 3).  
Figure 3: \(Cont.\)  
Figure 2: Changes in mean flow velocity (m\(\cdot\)s\({}^{-1}\)) in the monitored cross-sections along the Kwacza River before (T+0, white boxes) and after (T+6, grey boxes) restoration periods. Whiskers denote minimum and maximum velocities in a given cross-section, bars—confidence interval (\(\pm\)90%), horizontal line—mean velocity.  
The installation of various hydrootechnical structures contributed to greater fluctuations in parameter values along the watercourse. The coefficients of variation (_cv_) of DO and Chl-a, NH4\({}^{+}\)-N, TP, and Cl\({}^{-}\) increased (Table 4), and these parameters were most sensitive to changes in habitat conditions.  
The changes in WQ resulting from restoration activities on the Kwacza River were referenced to the hydrochemical background of the Supia River, the recipient of the Kwacza outflow. Both before and after the restoration, the mean PO4\({}^{3-}\)-P concentration was almost 3-fold higher in the Kwacza than in the Shupia, while COD and Chl-a concentrations were nearly 2-fold higher (Table 4). In the period before the restoration (T+0), the greatest differences were observed in the values of NO\({}_{3}\)\({}^{-}\)-N and NH4\({}^{+}\)-N, which were 7-fold and 4-fold higher in the Kwacza than in the Shupia, respectively. After restoration (T+6), only 2-fold differences were noted in the above mineral nitrogen forms. The concentrations of Ca and Cl were also significantly higher in the Kwacza than in the Shupia. The remaining parameters did not differ significantly between the analyzed periods.  
Figure 3: Changes in selected physical and chemical parameters of the restored section of the Kwacza River during two monitoring events in T+0 and T+6 (significant differences in the \(U\) test), and in the reference site on the Shupia River. The numbers below the x-axis denote the studied sites (Kwacza River: 1—site K1, 10—site K10; Slupia River: S).  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline  & \multicolumn{6}{c}{**Slupia River**} & \multicolumn{6}{c}{**Kwacza River**} \\ \cline{2-13} \multirow{2}{*}{**Parameters**} & \multicolumn{2}{c}{**T+0**} & \multicolumn{2}{c}{**T+6**} & \multicolumn{2}{c}{**Before Restoration**} & \multicolumn{2}{c}{**After Restoration**} \\  & \multicolumn{2}{c}{**N = 6**} & \multicolumn{2}{c}{**N = 6**} & \multicolumn{2}{c}{**T+0**} & \multicolumn{2}{c}{**T+6**} & \multicolumn{2}{c}{**T+6**} \\  & \multicolumn{2