Rivers are dynamic. They will, in time, change course through natural processes of erosion and sedimentation. However, different human activities can increase the rate at which these processes occur. This can have a significant impact on regional communities.
Floods can cause significant bed and bank erosion, often leading to loss of major public assets (e.g. roads and bridges) and damage to significant areas of private land. The impact on regional communities from these events can be devastating. For example, in the June/July 2007 floods in Gippsland the damage to waterways and flood-affected roads and bridges was estimated to be in excess of $66.5 million. Insured losses totalled $15 million but this is believed to greatly underestimate the losses to private individuals.
As a result of such a high potential for damage, there has always been great community interest in the condition and management of the river channel. River management in the past focused mainly on controlling bed and bank erosion and reducing the occurrence of floods and flood damage.
Thirty to forty years ago, sections of rivers were straightened and woody snags pulled out in the hope that this would increase the channel capacity, more effectively convey water and reduce flooding. The reality was that these activities often tended to increase bed and bank erosion, in some cases resulting in quite dramatic environmental damage.
Bed and bank erosion must be managed to protect high-value public infrastructure assets and private assets. Today there is a much better understanding within communities of the importance of the river channel for providing habitats for native fish and other animals.
The characteristics of a river channel vital to such habitats are:
- the composition of the bottom of the river is a mix of materials and shapes such as pools and riffles, cobbles, and sand
- the way the sides of a river channel are shaped can influence water depth, speed and quality as well as the presence of pooled areas and undercut banks
- the presence of large instream wood
- the presence of riparian (river bank and floodplain) vegetation
- that there is access for animals and plants, organic material and sediments to move both along the river and laterally into floodplains and wetlands.
River channel migration is the lateral motion of an alluvial river channel across its floodplain due to processes of erosion of and deposition on its banks and bars. In meandering streams, channel migration typically takes place by erosion of the cut bank and deposition on the point bar. In braided streams, channel change occurs due to sediment transport and the motion of bar forms through the channel.