## 4 Discussion  
River catchments in the temperate climate zone are characterized by high ground water levels and frequent spring floods; therefore, agricultural activities can compromise water quality in catchment areas [2; 46]. In farmland situated in the vicinity of rivers, fertilization and agricultural treatments contribute to uneven distribution of mineral compounds and contamination of surface and underground waters with nutrients, mostly nitrogen and phosphorus [8; 46; 47; 48; 49; 50]. Farmland is reclaimed to move excess surface water, which accelerates the runoff of chemical pollutants from fertilizers and anthropogenic substances, induces biochemical transformations in habitats, and influences water quality [51; 52]. These processes pose a particular threat for river catchments with mostly light or sandy soils. This is also the case in the Kwacza River basin where excess nutrients are easily discharged with rain water [45]. River sections where riparian vegetation is lost are more exposed to pesticides and fertilizers. These factors could be chiefly responsible for the decrease in ecosystem biodiversity [8; 48; 53]. In the past, water quality management was rarely taken into consideration in river restoration projects [54; 55]. The emphasis was placed on the restoration of habitat diversity and biotic communities in rivers and adjacent areas. The main goal of the Kwacza River restoration project was to stimulate the development of habitat heterogeneity in the riverbed along a 2.5 km-long section. During this process, mud deposits in the riverbed were replaced with sand to create favorable breeding grounds for the Atlantic salmon _Salmo salar_ L. and sea trout _Salmo trutta_ m. _trutta_ L., and to create slow-moving resting areas for migratory species [54]. In the present paper, we focused on the role of the installed hydrotechnical structures in improving river water quality. The restoration of the Kwacza River began in T+0, and the first measurable effects could be expected six years after the project's completion (Table 3). In the pre-restoration period, the removal  
Figure 5: Different variables were selected for the final GAM from the dataset, before (T+0) and after restoration (T+6). rates of NH\({}_{4}\)+-N, NO\({}_{3}\)\({}^{-}\)-N, PO\({}_{4}\)[3]\({}^{-}\)-P, Cl\({}^{-}\), and selected cations were low, and NO\({}_{2}\)\({}^{-}\)-N and TP concentrations increased in the studied section of the river (Table 5). After restoration, hydrotechnical treatments and greater insolation enhanced river self-purification processes. The above can be attributed to greater differences in the morphological parameters of the river channel as well as the introduction of macrophytes, which substantially decrease flow velocity and contribute to self-purification processes [1; 52; 55; 56]. The applied treatments, including groynes, semi-palisades, and stone islands, also increased DO levels and promoted the decomposition of organic matter by microorganisms. Unlike P, N can be permanently removed by coupled nitrification-denitrification, a process that transforms inorganic compounds (NH\({}_{4}\)\({}^{+}\), NO\({}_{2}\)\({}^{-}\), and NO\({}_{3}\)\({}^{-}\)) into inert gaseous products (N\({}_{2}\)O and N\({}_{2}\)) [23]. Our results indicate a transition from locally anoxic conditions with increased NH\({}_{4}\)\({}^{+}\)-N concentrations during the pre-restoration period, and a decrease in NH\({}_{4}\)\({}^{+}\)-N levels with an increase in DO after restoration. This change seems to indicate that locally higher water velocities stimulate an increase in DO, whereby mineralization and nitrification are responsible for the conversion of NH\({}_{4}\)\({}^{+}\)-N to NO\({}_{3}\)\({}^{-}\)-N. The restoration of the riverbed with river bars and groynes promoted the growth of aquatic vegetation, and effectively contributed to a decrease in NO\({}_{3}\)\({}^{-}\)-N due to plant uptake, which was accompanied by an increase in Chl-a.