Alteration of natural flows
The natural flow in a river continually changes, mainly due to the variability of rainfall. Many of these changes are important for the growth and regeneration of trees and plants on the floodplain, and for the breeding habits of fish, waterbirds and other animals.
For example, under natural conditions rivers generally have lower flow in summer and autumn, and higher flow in winter and spring. Small variations within this pattern are also very important. In summer there may be higher-than-normal freshening flows, and in winter and spring rivers may overtop their banks and fill billabongs and other channels. Wetting of floodplains during spring may be important for tree growth, while the first high flow in spring can stimulate fish to go upstream and breed.
Native plants and animals have evolved to this variability in flow and are best suited to these sorts of conditions. When dams are built and water is extracted from rivers, the characteristics of these natural, variable flow regimes are altered. When water is extracted and used, rivers with no major dams can suffer from regular extreme low flows in summer.
These changes in natural flow regimes can have significant impacts on the number, diversity and health of native plants and animals, and their ecosystems.
When large dams are present water is generally collected in winter and released in summer, resulting in very low flow in winter and constant high flow in summer. This is the reverse of natural conditions to which our ecosystems have evolved. This interferes with the breeding habits of the fauna and can lead to a real reduction in populations.
Problems can be compounded by the presence of introduced species, which may prosper under the changed conditions.
Discussion of changes to flow regimes in the Murray Darling Basin and the consequences are available.
More information about the above issues is available from the Department of Sustainability and Environment's website.