The Struga Kwilecka is flowing through several lakes, watershed is dominated by arable lands and its total area is equal to 48 km2. Studies were carried out in part of river localised in a forest. Analyses of water quality changes of watercourse below sewage discharge were made in the years 2008–2011. Waste water treatment plant situated in Kwilcz town is a biological type one with additional settling pond, which sometimes caused increase of phosphorus concentration in sewages discharged to studied watercourse. Worsening of river water quality was found particularly in site localised 100 metres below discharge. In other sites situated 350 and 700 metres below interceptor the situation was getting better despite low river water discharge. Positive changes in water quality were observed in case of trophic parameters, as like as soluble reactive phosphates, total phosphorus, nitrates and also for conductivity. Due to shadow conditions in river valley only few aquatic species were identified in watercourse with domination of reed with some minor area covered by other taxa including mosses.