The River Continuum Concept (RCC) describes the entire river system as a continuously integrating series of physical gradients and associated biotic adjustments as the river flows from headwater to mouth.
Within the stream system, longitudinal connectivity refers to the pathways along the entire length of a stream. As the physical gradient changes from source to mouth, chemical systems and biological communities shift and change in response. The River Continuum Concept can be applied to this linear cycling of nutrients, continuum of habitats, influx of organic materials, and dissipation of energy.
For example:
- A headwater woodland stream has steep gradient with riffles, rapids and falls.
- Sunlight is limited by overhanging trees, so photosynthesis is limited.
- Energy comes instead from leaves and woody material falling into the stream
- Aquatic insects break down and digest the terrestrial organic matter.
- Water is cooled by springs and often supports trout.
In the mid-reaches,
- the gradient decreases and there are fewer rapids and falls.
- The stream is wider, sunlight reaches the water allowing growth of aquatic plants.
- Insects feed on algae and living plants.
- Proportion of groundwater to runoff is lower so stream temperatures are warmer.
- The larger stream supports a greater diversity of invertebrates and fish.
The river grows and the gradient lessens with few riffles and rapids.
- Terrestrial organic matter is insignificant in comparison to the volume of water
- Energy is supplied by dissolved organic material from upstream reaches.
- Drifting phytoplankton and zooplankton contribute to the food base as does organic matter from the floodplain during flood pulses.
- Increasing turbidity reduces sunlight to the streambed causing a reduction in rooted aquatic plants.
- Backwaters may exist where turbidity has settled and aquatic plants are abundant.
- Fish species are omnivores and plankton feeders such as carp, buffalo, suckers, and paddlefish.
- Sight feeders are limited due to the turbidity (MN DNR, Healthy Rivers).
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The river continuum concept emphasizes the longitudinal dimension of the stream ecosystem. The RCC proposes a progressive shift, from headwaters to mouth, of physical gradients and energy inputs and accompanying shift in trophic organization and biological communities (Vannote et al, 1980, graphic Stream Corridor, FISRWG).