A river system consists of a branched network of streams, but how

do environmental conditions and the distribution of organisms vary

along the system? The depth, width, discharge and mean age of the

water increase from source to mouth and the physical and chemical

characteristics of the water gradually become more infl uenced by

contact with the atmosphere and in-stream processes. Species composition

and diversity also change along the stream. These patterns

are infl uenced by the location and site-specifi c characteristics of the

reaches. In addition, they are infl uenced by the dispersal of organisms

between reaches making the biota at each site dependent on

the biota at neighbouring sites.