Streams and rivers are enormously important ecologically, economically, recreationally, and esthetically. This importance far outweighs their proportional significance on the landscape. Running waters constitute less than 1/1000th of the land surface and of freshwater resources of Earth and contribute only 2/10 000th of annual global freshwater budgets. Streams and rivers are significant agents of erosion and serve a range of human needs, including transportation, waste disposal, recreation, and water for drinking, irrigation, hydropower, cooling, and cleaning. At the same time, flooding of streams and rivers pose potential natural hazards to human populations. Irrespective of their impact on man, streams and rivers are rich, complex ecosystems that are diagnostic of the integrity of the watersheds through which they course.

There has always been a general anecdotal notion of what constitutes a stream and what constitutes a river; that is, streams are small, narrow, and shallow while rivers are large, wide, and deep. However, the difference between them is without clear distinction in the literature of the last 100 years. For the purposes of this article, streams refer to channels in drainage networks of orders 0-5 and rivers as orders 6-12 and above (see definition of stream order under the section titled 'Channel morphology'). In this article, the history of stream ecology is discussed followed by a treatment of the physical and chemical setting and biological features of major groups of lotic organisms. In a companion article, ecosystem dynamics and integrating paradigms in stream and river ecology are covered.

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