LONG-TERM EFFECTS OF RIVER REGULATION ON RIVER MARGIN VEGETATION

(1) The effects of river regulation on river margin vegetation were evaluated by comparing two parallel seventh order rivers, one natural and the other strongly regulated, in northern Sweden. Prior to regulation, both rivers had similar vegetation. (2) No difference between the natural and the regulated river was found in width and height (relative to the summer low-water level) of the river margin, number of substrates, and mean annual discharge. (3) Frequency distributions of species differed in that the regulated river had fewer frequent and more infrequent species. Species-richness and the percentage cover of vegetation were both lower per site in the regulated river. The proportion of annual plus biennial species-richness was higher and that of perennial species-richness lower along the regulated river. (4) Reservoirs retaining pre-regulation river margins and remnants of their former vegetation, and stretches with a modest flow regulation, were most floristically similar to the natural river. (5) Regression equations including eight independent variables explained 10-77% of the variation in species-richness in thirteen groups of plants and in plant cover for two vegetation layers. Presence of pre-regulation river margin vegetation, water-level regime, height of the river margin, and mean annual discharge were the most important variables for species-richness, while water-level regime, mean annual discharge and substrate fineness were most important for plant cover. (6) In most cases, values of species-richness were higher in natural sites and in regulated sites with remnants of pre-regulation river margin vegetation, whereas they decreased with increasing height of the river margin. Percentage cover of ground vegetation was highest in natural sites with a fine-grade substrate.