The study area was a small lowland river valley (the Łęg river) located in the south-east of Poland. The object of investigation was the macrophytes of 10 river lakes with corresponding active river channel stretches of the same length as the cut-offs. The aim was to check the difference in species diversity between cut-off and active river channels. The second aim was to test the following hypothesis: vegetation of river lake has been shaped under the influence of contiguous river (...) stretch which has left a measurable mark in species abundance and composition. To test this, we checked whether a cut-off channel’s flora is more similar to flora of the contiguous river stretch, than to flora of a farther river stretch. During the course of the study it was found that the average species richness was approximately two times higher in the cut-off channels than in the river stretches. The number of the species exclusive for the river lakes was nine times higher in comparison with the river’s exclusives (not found in the cut-offs). The Shannon diversity index definitely spoke in favor of the river lakes. These results clearly show the significance of river lakes in maintaining biodiversity of aquatic ecosystems in a small river valley. We did not confirm our hypothesis concerning floristic relation between an active channel and its cut-offs. The floristic similarity between a given cut-off channel and the contiguous active river channel stretch is not stronger than the similarity between this lake and more distant river stretches. The cause of such a state of affairs may be high natural dynamics of investigated habitats and anthropogenic transformation of the river valley. (shrink)