Catchment areas of Icelandic rivers are mostly barren or with little vegetation cover in the highlands, but with heathland and mire vegetation in the lowlands. Chemical composition and nutrient availability in Icelandic rivers are influenced by geology, topography and vegetation cover in the river basins. This seems to determine the density and diversity of benthic invertebrates, species composition of anadromous fish and catch of salmon in Icelandic rivers. Species composition of benthic communities is determined by particulate organic matter drifting downstream from river head-waters. Filter feeding blackfly larvae dominate lake outlets, while algal grazing chironomid larvae dominate rivers not influenced by lakes. In well vegetated catchment basins, lake-fed rivers have higher catches of salmon than non-lake fed rivers. Only a few of the rivers flowing from poorly vegetated areas sustain salmon. Glacial rivers have the lowest density and diversity of benthic invertebrates of all river groups and do not sustain fish populations.