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The surveys made on 80 river sites in Ecoregion 14 (Poland) were undertaken in years 2006-2008 to determine influence of river modifications and water trophy on variability of taxonomic structure of macrophyte taxa. Field surveys were conducted using widely accepted methods such as the River Habitat Survey and Mean Trophic Rank, supplemented by physico-chemical analyses of water and hydrochemical index for evaluation of trophic level. Obtained results showed, that there are significant differences between lowland and upland river sites according to the rate of channel modifications, concentration of trophic parameters and thus between aquatic macrophyte structure. The variability of taxonomic structure of aquatic macrophytes was found according to site altitude, rate of modifications and water quality parameters. It was found, that in case of lowland rivers the simultaneous influence of modifications and water trophy can affect taxonomic structure stronger than in upland sites, where level of kinetic energy of water flow plays the most important role. [ABSTRACT FROM AUTHOR]