Individual streams are part of a larger network called a watershed.

A watershed begins with small trickles of water. Many of these small trickles gather and form consecutively larger stream types which grow: from brooks to creeks to rivers. As the volume of water increases in the stream channel, the stream channel has a greater cross-sectional area.

High Elevation

Valley

Lower elevation streams have both larger cross-sectional areas and greater volumes of water than mid- and high-elevation streams.

There is a pattern of progressive physical change that occurs from higher in a watershed to the base. The changes include :

• increase in stream size - width, depth, velocity, and flow volume
• decrease in shading (canopy cover) by the riparian zone
• decrease in size of streambed substrate; from boulders in mountain streams to sandy-bottomed rivers.

These changes occur due to geology. In the higher elevations, streams are narrow, soils are thin and streambeds are mostly rock--not much sediment. As a stream flows and grows, it picks up sediment and flattens out.

Living is not easy for aquatic creatures at either end of the continuum-- in the headwaters or at the lower reaches of the river. The environment is harsh: less space, less food, greater extremes of temperature. It's in the middle of the continuum where they find more opportunities to make a suitable living.

Stream-dwelling communities, such as fish and aquatic invertebrates, respond to the different changes. The "River Continuum Concept" is trying to help describe the physical processes (geology, climate) outside of a river that effect the biological processes (vegetation) along a river, which in turn effect the physical and biological processes within a river (temperature, nutrients). In other words, the pattern of physical change in a watershed and the biotic adjustments are called the "River Continuum Concept".