The aim of the research project was to recognise the degree of variability of the mineral composition of the Kończak River and its tributary the Połajewski Canal set against the plant cover of its valley. The variability of the plant cover and the intensity of agricultural cultivation occurring along the examined water courses were determined on the basis of the studying of maps and inspection of the area. As a result, four plant complexes were identified along the Kończak River and two other ones along the Połajewski Canal. Systematic analytical investigations of water quality were conducted during the vegetation period of the year 2000. Places from which water samples were collected coincided with the boundaries of individual complexes. Concentrations of the following most important parameters, from the point of view of mineral composition, were determined in water samples: nitrate nitrogen, phosphorus, magnesium, calcium, sodium, iron, manganese, copper and zinc as weil as chlorides and sulphates. Results of our investigations indicate that waters of the Kończak River and its tributary, the Połajewski Canal, meet the current requirements of the 1st class of purity of surface waters with regards to the determined mineral constituents. Areas directly adjacent to both watercourses are covered, primarily, by permanent plant covers. The prevalent cover is that of forests (mainly along the Kończak River) with pine as the dominant tree species. Permanent meadows cover considerable areas, mainly along the Połajewski Canal. On the other hand, the share of arable land is negligible. There is no doubt that permanent plant covers occurring along these watercourses exert a strong influence on concentrations of the examined biogens in these waters. Therefore, these communities should not be replaced by arable land. Permanent meadows occurring here are dominated by communities with Arrhenatherum elatius, Phalaris arundinacea and Holcus lanatus. One of the unusual phenomena is the occurrence of areas with large proportions of Trisetum flavescens forming a Trisetetum flavescentis association, which is very rare on lowlands, also in Wielkopolska. It is desirable, for forage, ecological and landscape reasons, to preserve the plant cover currently existing along the Kończak River and Połajewski Canal. This should be the main purpose of agricultural, grassland and forest management practices.