Summary:

This paper presents the ecological risk factors related to the watercourses regulation and maintenance works on the small and medium-sized lowland water-courses found in Lower Silesia. These factors include: occurrences related to the execution of regulation and maintenance works, which have an impact on the eco-logical condition of the watercourses, probability of these occurrences and their consequences in the form of adverse ecological changes in the river beds. Based on the observed responses of aquatic vascular plants species to the technical inter-ference on a watercourse's bed, a detailed assessment of one of these factors - ecological changes in the watercourse's bed was made. Field studies were con-ducted during 2007-2008 on 44 sections of various unregulated, conserved and regulated watercourses. Research included identifying the species of aquatic macrophytes found in the streambed as well as determining the degree of cover-age.
The field studies showed, that the regulation and maintenance works had an adverse impact on the aquatic plant species composition. In most cases species removed during the technical interference did not regenerate. In some transformed study sections the plant species abundant was lower than on the unregulated tran-sects. Only in rare cases the plant species abundant in the compared study sec-tions was similar. The results show, that in this paper examined risk factor - eco-logical effects of works, is generally at a high level. The consequence is a high level of ecological risk in the transformed watercourses. Therefore for each project related to technical interference in the watercourse's bed the ecological risk management plan should be drawn up.