A quantitative analysis of grasses (Poaceae family) growing in vegetation alongside watercourses in rural regions of Poland’ Odra Valley was undertaken. The quantity of a given species was calculated by multiplying the length of sections of the watercourse by the percentage coverage of this species. In total, 134 km of watercourses were analyzed. Among the 164 plant species found, 22% were grass species. Its quantitative participation was 39%. The most abundant species was Calamgrostis epigejos followed by Phragmites australis. Analysis of phytosociological affinity revealed the importance of the Molinio-Arrhenatheretea class and a considerable amount of species from Querco-Fagetea. The majority of grass species were apophytes; only 12% were antropophyte. The dominance of native species, which form specific semi-natural assemblages in human-disturbed habitat, highlighted the function of riparian vegetation in maintaining biodiversity as well as ecosystemal services in modern agricultural landscapes.