A natural channel differs from a prismatic one by its rich variability. Regulated rivers have a geometrically regular shape that is not suitable for the instream biota. The morphology of a regulated river is the main cause of habitat changes. The article provides the results of research that was focused on assessing the impact of a channel's morphology on the instream habitat preferences by ichthyofauna. In the article the ichthyofauna is represented by brown trout (Salmo trutta m. fario). During the years 2015 and 2016, research was carried out for 13 reaches of mountain watercourses in Slovakia. These reaches were geodetically measured, and a hydraulic model was created. The ichthyological survey was focused on the fish's habitat preferences. A set of data was created from the results that was statistically evaluated by cluster analysis, and the correlation relations for the abiotic and biotic characteristics of the watercourses were evaluated. The individual evaluations provide mutually compatible results. It can be concluded from the results that bioindications by fish provide reliable results in relationship to the changes in habitats in regulated rivers. There was an unambiguous statistical correlation in the preferences of brown trout in the regulated and natural rivers.

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Globally, invasive species represent a serious threat to biodiversity and to the ecosystem. As an undesirable part of riparian ecosystems, invasive plants form continuous growths on the banks of watercourses. One of the biggest problems at river bank sites is Japanese knotweed, Fallopia japonica (Houtt.) Ronse Decr, which is an extremely invasive and aggressive weed. The topic of the interaction of flow and invasive plant species in shore stands is rich and presents a wide range of possibilities for research. The presented paper brings the results of our studies on the invasions of F. japonica in chosen riparian vegetation sites in Austria and Slovakia from 2011–2020 (36 stands). Our research was aimed at the survey on the changes in the distribution (spread) of F. japonica at the selected river sites; assessment of the impact of the watercourse regime on the spread of F. japonica; monitoring of the population growth dynamics of F. japonica and assessment of possibilities for effective eradication of F. japonica in context of the riparian vegetation management. We used standard techniques of field survey, mapping, flow modeling/simulation, and laboratory experiments. Our research results showed that water streams are not primary invasion starters until there is a flood. As long as flooding does not exceed the critical speed of the water stream, there is no direct damage to the invasive plant. The water body can be a secondary trigger for plant invasion at normal speeds. In addition, F. japonica reproduction ability directly conditions its population dynamic growth. We can report that selective invasion removal adapted to local conditions can be most suitable and beneficial for municipalities.

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