## 3 Results  
### Hydrological Characteristics  
The hydrological conditions observed at the studied sites in 2007 and 2013 were characteristic of river networks with a complex hydrological regime in the temperate zone. The mean discharge of the Slupia River at the gauging station in Slupsk was estimated at Q < 14 m\({}^{3}\).s\({}^{-1}\). In spring, discharge usually increases due to melt water, it increases in summer due to rainfalls, and is minimal in autumn. Overflows and backward flows from the Shupia to the Kwacza were not observed in the analyzed periods. Before the restoration project, the hydrological regime of the Kwacza River was marked by ground-rain-melt supply with an average flow of 0.41 m\({}^{3}\).s\({}^{-1}\) (Table 3).  
Overland flow was similar to other rivers in that region, and it was estimated at 6.0 L-s\({}^{-1}\).km\({}^{-2}\). No significant changes in mean water stage were observed along the restored section. In the T+6 period, both water flows and water stages were somewhat lower in sites K1 and K10 relative to T+0, which could be associated with the draining effect of the Slupia River during low water stage, especially in the summer of 2013.  
The applied hydrotechnical structures generally decreased flow velocity along the restored section of the Kwacza River, and water retention time increased after the reconstruction of the river's cross-sectional geometry (Table 3). The most significant changes in water flow were observed in sites where double groynes (K6) and stone islands (K8) had been introduced (Figures 1 and 2).  
The narrowing of the river channel increased channel depth and maximum flow velocity in the cross-section to 0.9 m\(\cdot\)s\({}^{-1}\). The applied structures induced clear changes in streamflow velocity (\(U\) test, \(p\) < 0.01). In the T+0 period, mean flow velocity was higher than in the T+6 period only at sites K1, K2, and K10 (1.5-fold and 6.5-fold, respectively) (Figure 2). In the remaining sites, restoration works contributed to a significant increase in water velocity. The highest velocities were noted in the cross-section with stone islands (K6) where water current reached 1.2 m\(\cdot\)s\({}^{-1}\), and in the double groynes (K5) and double semi-palisade (K7) where water current increased to 1.1 m\(\cdot\)s\({}^{-1}\). Both the groynes and the semi-palisade contributed to significant stabilization of water levels.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Cross-Sections**} \\ \cline{2-5}
**Parameters** & **K1 (A = 63 km\({}^{2}\))** & **K10 (A = 85 km\({}^{2}\))** \\ \cline{2-5}  & **T+0** & **T+6** & **T+0** & **T+6** \\ \hline Streamflow Q\({}_{\mathrm{mean}}\) (m\({}^{3}\).s\({}^{-1}\)) & 0.41 & 0.38 & 0.55 & 0.48 \\ low flow Q\({}_{\mathrm{l}}\) (m\({}^{3}\).s\({}^{-1}\)) & 0.07 & 0.09 & 0.09 & 0.12 \\ high flow Q\({}_{\mathrm{h}}\) (m\({}^{3}\).s\({}^{-1}\)) & 1.53 & 1.40 & 1.39 & 1.45 \\ Volume V (10\({}^{6}\).m\({}^{3}\).year\({}^{-1}\)) & 12.93 & 11.98 & 17.34 & 15.14 \\ Overland flow q (L-s\({}^{-1}\).km\({}^{-2}\)) & 6.55 & 6.10 & 6.51 & 5.68 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Hydrological changes in the Kwacza River before (T+0) and after (T+6) restoration at cross-section sites K1 and K10; A—catchment area. Data given in the table represent mean annual values.