The Murray-Darling Basin
The Murray-Darling Basin covers one-seventh of Australia and produces 40% of our agricultural produce. It is Australia's food bowl, where 70% of our irrigation takes place and 42% of our farm land is located. It is fed by 408,000 kilometres of rivers.Australia's average rainfall is 455 millimetres. Half of the rainfall lands on just a quarter of the land area, a fringe around the eastern and northern coasts. Only 6% of our rain falls in the Murray-Darling Basin.
Water use and effects
We remove around 11,500 gigalitres (one gigalitre is rougly equal to 500 Olympic swimming pools) of water from the Murray and Darling Rivers per year, of its average total of about 14,000 gigalitres. Irrigation accounts for 95% of the water used. The Murray River supplies about 40% of Adelaide's drinking water supply.Where irrigation has been used, the water table has risen, bringing with it tonnes of salt. It is now thought that between three and five million hectares of the basin will be salt-affected within this century. Irrigation can also damage the soil structure and degrade water quality. We have changed the natural high river flows from spring to summer and autumn, upsetting the natural breeding rhythms of many species.
Along about 1,000 kilometres of Murray riverbank and floodplain thousands of river red gums are dying from lack of water. Natural flooding once regularly soaked these trees, every 3.3 years. Now these trees have not seen a flood since 1993. It is estimated that 50 - 80% or wetlands in the Basin have been destroyed.
The reduction in the flooding of the floodplains has also devastated native fish and bird populations. Native fish populations have fallen to 10% of their original numbers. One third to a half of fresh water fish across the Basin are threatened with extinction Australia's largest freshwater fish, the Murray cod, has been added to the national list of threatened species. Numbers of cod have declined by 30% in the last 50 years.
Dams and weirs
There are 30 dams and 3,500 weirs in the Murray-Darling Basin. Nearly three times the annual average flow in the Murray River is stored in dams and weirs. The dams and weirs prevent fish movement up and down-stream for seasonal breeding and feeding. They also replace flowing river conditions with still, lake-like conditions, changing the essential habitat of many plant and invertebrate species.The weirs have also provided an ideal habitat for toxic blue-green algae and European carp, which are rapidly displacing native fish.
The Murray mouth
Less than 20% of its capacity now flows out of ther River Murray Mouth. The frequency of no flow at the mouth has increased from 1 in 20 years to 1 in 2 years. Median annual flows to the sea are only 27% of the natural flow.Only 11% of the natural estuary at the Murray Mouth remains, most of it having been divided off by five 'barrages'. The internationally listed estuarine wetland, the Coorong, has lost 90% of the migratory wader birds which once inhabited it.