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 Corrico
Connectivity refers to the flow, exchange and pathways that move organisms, energy and matter throughout the watershed system. These interactions create complex, interdependent processes that vary over time.
As with hydrology, stream connectivity can be described in four dimensions:
- longitudinal – linear connectivity
- lateral – floodplain connectivity
- vertical – hyporheic (below the stream bed)
- temporal (time) – many scales; seasonal, multiyear, generational
Additionally, the concept of landscape connectivity expands on this idea to include the entire watershed ecosystem as connected by the flow of organisms, energy and nutrients.
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 (RCC) can be applied to this linear cycling of nutrients, continuum of habitats, influx of organic materials, and dissipation of energy.
- 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).
Lateral connectivity allows the stream access to its floodplain during high water events. This access is critical for the healthy ecosystem function. Nutrients and organic matter are transported to the stream from the floodplain, plant and wildlife species flourish in the diverse successional stages of inundated areas, and aquatic species gain access to seasonal habitats essential to their life cycles.
Lateral connectivity refers to the periodic inundation of the floodplain and the resulting exchange of water, sediment, organic matter, nutrients, and organisms. Lateral connectivity becomes especially important in large rivers with broad floodplains.
Periodic floods refill oxbow lakes and recharge wetlands. Inundated areas may be used as spawning areas by species such as northern pike. Floodwaters carry nutrients and organic matter from the land to the stream's aquatic plants, plankton, stream invertebrates, and fish. Seasonal flooding produces a variety of streamside vegetation and habitat for a diversity of birds and mammals (MN DNR, Healthy Rivers).
Access to floodplain is also important for small streams that can experience dramatic episodic flooding. Heavy, localized rains can cause small streams to rise several feet in a few hours. This flashiness is largely a result of more overland flow and less infiltration following the conversion of native land cover to row crops and human communities. Large amounts of sediment are mobilized by these events, impacting all trophic levels and altering biological communities in the stream and the adjacent floodplain.
Mixing of surface water and ground water occur in the hyporheic zone. This biologically active zone contains water percolating through the permeable soils adjacent to the open streambed. Important microbial activity and chemical transformations are enhanced in this area (Stream Corridor, FISRWG).
Vertical connectivity is represented by the connection between the atmosphere and groundwater. The ability of water to cycle through soil, river, and air as liquid, vapor, or ice is important in storing and replenishing water. This exchange is usually visualized as unidirectional–precipitation falling onto land and then flowing over land or percolating through the ground to the stream.
An equally important transfer of water occurs from the streambed itself to surrounding aquifers. Groundwater can contribute to flows in the river at certain times in the year and at certain locations on the same stream. Streams may either gain or lose water to the surrounding aquifer depending on their relative elevations. Lowering the water table through groundwater withdrawals may change this dynamic exchange in unanticipated ways (Stream Corridor, FISRWG).
The slow movement of water through sediments to the river produces several ecological benefits.
- The water is filtered of many impurities.
- It usually picks up dissolved minerals.
- The water is cooled.
- The water is metered out slowly over time.
This is particularly important in smaller, cooler streams for the maintenance of critical habitat for fish, wildlife and invertebrate species.
A stream exhibits temporal connectivity of continuous physical, chemical, and biological interactions over time, according to a rather predictable pattern. These patterns and continuity are important to the functioning of the ecosystem. Over time, sediment shifts, meanders form, bends erode, oxbows break off from the main channel, channels shift and braid. A stream rises and falls according to seasonal patterns, depending on rain and snowmelt. Throughout most of Minnesota, free-flowing rivers experience high water in spring, falling flows in summer, moderate flows in fall, and base flows in winter. The watershed has adjusted to these normal fluctuations, and many organisms have evolved to depend on them (MN DNR, Healthy Rivers).