Riparian land is a particularly dynamic part of the landscape. That is, it can change markedly — even under natural conditions.
Fires, frosts, cyclones and floods can all have large impacts on riparian lands and result in major changes to channel position and shape, as well as to the surrounding vegetation.
Human impact since European settlement is very noticeable on riparian lands, with large-scale changes in condition and health. In southern Australia, there has been the widespread removal of riparian vegetation for agricultural or urban development. Northern Australia is fortunate in that clearing has been less widespread, although there are many areas where most natural vegetation (including riparian vegetation) has been removed, especially along the coastal lowlands.
The degradation of riparian land is often associated with the removal of native vegetation. The major impacts are summarised as follows:
- Removal of riparian trees increases the amount of light and heat reaching watercourses. This favours the growth of nuisance algae and weeds, and can completely change the in-stream ecosystem to one that no longer supports native fish and other animals. Excessive in-stream weed and algal growth can trap bed sediment, leading to blockage of the channel and a reduction in its capacity to convey flood waters.
- Under natural conditions, trees occasionally fall into watercourses, creating woody debris — an important habitat for animals and plants living in the stream. Removal of this debris and of the source of large branches and trunks disrupts these in-stream ecosystems by reducing habitat for fish, crustaceans and other animals.
- Continuation of agriculture to the top of streambanks increases the delivery of sediments and nutrients to watercourses. Large volumes of fine-grained sediment smother in-stream habitat and ‘clouds the water’, while increased nutrients stimulate weed and algal growth. Increased sediment and nutrient load also affects estuarine and marine life beyond the river mouth.
- Removal of riparian vegetation and death of its protective root systems destabilises streambanks, often resulting in large increases in channel width, channel incision and gully erosion. This erosion of the channel often delivers more sediment to streams than does human activity on the surrounding land. Significant areas of valuable agricultural land can be lost, and water turbidity is increased.
- Removal of vegetation throughout the catchment can lead (and has led) to raised water tables and salinisation of land which, as salt levels increase, drains into rivers and streams and results in saline watercourses.
The removal of vegetation is not the only human landuse that adversely affects riparian land:
- Alteration of water movement (through the imposition of dams, weirs and pumps directly within streams) can severely affect instream populations and the capacity of the watercourses to carry flow. Reduced flow below dams, rapidly changing water levels due to dam releases, and cold water from deep off-take points, all reduce the health of riparian vegetation and in-stream plants and animals.
- Sand and gravel removal, channel straightening, and construction of levee banks and drains, can all result in channel incision and headcutting which, in turn, increases bank height and slope, and leads to increased erosion rates, loss of agricultural land and damage to infrastructure such as roads, bridges and buildings.
- Uncontrolled access by stock to riparian areas leads to grazing and trampling of vegetation, breakdown of soil structure and contamination of the water with nutrient-rich urine and faeces.
- Altered fire regimes and invasion by exotic weeds also degrades riparian land.
- Urban development, common in coastal catchments, is also a major influence on water quality and the condition of riparian areas.
The impacts of these disturbances are not only cumulative; they exacerbate each other. For example, clearing riparian vegetation from tributary streams multiplies, many times, the impact of increased nutrients. This is because clearing also provides the light and higher temperature conditions needed to enable nuisance weeds and algae to flourish in the stream and to dominate the in-stream ecosystem.
NOTE:
Sourced from Adelaide & Mount Lofty Ranges Natural Resource Management Board