River Ecosystems

River ecosystems drain the landscape through hierarchical series of fluvial channels, beginning with small headwater streams, and enlarging, ultimately, to estuaries meeting the sea. Several conceptual models that provide unifying concepts about the connections of rivers with the landscape in terms of ecosystem properties such as processing of energy and matter, habitat, biodiversity, and resilience in the face of disturbance are discussed. Major groups of riverine biota are described. The results of human activities, including climate change, which pose threats to river ecosystems, are briefly reviewed. These include placing land from forests, grasslands, and wetlands into urban or agricultural uses; damming of rivers and streams; pollutant loading from urban; industrial and agricultural sources; alteration of natural drainage characteristics; introducing new species; overharvesting of fish and other species; and the numerous impacts of climatic change on hydrology and riverine biogeochemical cycles.