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River phenomenon describes distinctive, deeply incised river valleys in the landscape of Central Europe in the sense of their biotic, geomorphologic and climatic features. This term is used mainly in the Czech Republic, where it was introduced by several studies in 1960s. Distinctiveness of deep river valleys is based on their sharp contrast to softly undulating landscape, prevailing in middle elevations of Czech. Steep slopes, exposed rocks pronouncing the effect of geological substrate on vegetation, contrast between the sunny and warm southerly oriented slopes and shaded cold northerly oriented slopes, high diversity of various extreme habitats situated next to each other, specific microclimatic conditions causing frequent temperature inversions, the main feature for existence of river phenomenon. The main biotic consequences are: (1) high biodiversity located in deep river valleys, (2) concentration of relict species, resulting from the fact, that deep river valleys served as a refugia for plant and animal species during the era of ice age, (3) function of river valley as a corridor for migration of plants and animals, connecting mountain areas with lowlands, and (4) preservation of the vegetation relatively unaffected by human activities for its inaccessibility.
Although river phenomenon has been applied mainly to Central Europe, it could be related to riverine landscapes, with the view focused specifically on rivers with deep, narrow valleys.