Organism Movement

The timing of some animal movements may be predictable – reflecting daily or seasonal movements – but vary according to species. Seasonal spawning migrations can be affected by environmental conditions such as water temperature or velocity. Other movements are in direct response to changing conditions (food availability, temperature, oxygen levels, water levels and flow velocities) either to take advantage of opportunities (access to floodplains) or to avoid adverse conditions.

For all animal species within river and stream ecosystem movement is essential for populations to persist. Movements may be between areas of shallow and deeper water or between the water's edge and mid-stream areas. Animal movements may be downstream (intentionally or unintentionally) or upstream. For many organisms inhabiting small streams, lateral movements or movement between surface and deeper water within the stream channel are severely constrained. Under these circumstances upstream and downstream movements are all the more important for aquatic organisms. Also important are movements between the stream channel and adjacent floodplains, as well as upstream and downstream through floodplains and riparian areas. For rivers with large floodplains these movements are especially important.

Some organisms are capable of moving only relatively short distances unless displaced by floods or when attached to other animals or woody debris. Others – such as migratory fish – are strong swimmers with the capacity for long-distance movements and the ability to move upstream against strong currents. In between, are a whole host of species: some with the capacity for strong bursts of swimming but with a tendency to stay put, and others – some crayfish for example – that are capable of long-distance movements but typically crawl rather than swim.

Many weak swimmers and crawling species take advantage of boundary zones along bank edges and the stream bottom where water velocities are much lower than in the water column. Under natural conditions movement by some stream organisms depends on the diversity of channel structure and hydraulics typically found in natural streams. This diversity creates alternate pathways throughout the channel bed and along the bankline; if any point in the channel is a barrier (e.g. high velocity, hydraulic jump) other less strenuous pathways are generally available. Maintenance of unfragmented stream bottom and bank edge habitats is the best strategy for maintaining continuous and interconnected populations for a variety of weak swimming species.