Studies were carried out in a wide flat valley located in the middle course of the Krasna River, which is flooded during excessive rainfall. The whole flood plain is dominated by the communities of hydrophyte plants, e.g. Phragmites australis, Phalaris arundinacea, Carex gracilis, Equisetum limosum, Molinia coerulea, Alopecurus pratensis, Holcus lanatus and others. After the inundation of 1997, surveys of the main plant communities were made in 1998 according to the Braun-Blanquet method and were repeated in 2002 on selected plant communities. The results show that the botanical composition of plant communities changed: the proportion of hydrophytes decreased and that of mesophytes increased in the sward, and even some xerophytes appeared. Due to those changes the river channel became overgrown, which hampered the outflow of surface waters from the valley. Another factor contributing to the increased bogginess of the valley was the improper operation of the weir damming up water for a local industrial plant.

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