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We studied bank vegetation between the spring high—water level and the summer low—water level along two rivers in northern Sweden. The hypothesis that natural and ruderal species would show different downstream patterns of species richness was tested by sampling species composition and environmental variables along 200 m long stretches of riverbank 10 km apart. Natural species richness was highest in the midreaches of both rivers, whereas ruderal species showed a significant, monotonic increase downstream. There are no obvious mechanisms producing the quadratic pattern of natural species richness. The downstream increase in ruderal species suggests a founder effect depending on larger artificial disturbances near the coast, but alternative explanations are also given. Total species richness did not exhibit any interpretable downstream patterns. The only factors significantly correlated with total …