The Murray-Darling Basin is a very important river catchment in Australia. It covers a seventh of the land area of our continent, produces 40% of the nation's agricultural profit, and 70% of our irrigated agriculture. Yet much of the basin is arid, and much of the agriculture relies in rains falling in the eastern ranges and flowing hundreds of kilometres before it is pumped from the river.
But almost 200 years of settlement, agriculture and land clearing, and a century of irrigation, have left their mark. Notable changes to the natural ecosystems include:
- Many of the basin's rivers have had their flows reduced to a trickle at times, changing the riverine ecosystems. Wetlands at the downstream ends of most rivers (including the important Macquarie Marshes) do not receive nearly the required amount of water to sustain large bird populations. And flows to the Coorong, an enormous estuarine area where the Murray reaches the sea, have been so reduced that the ecology is becoming more saline and the whole character of this water body is in danger.
- Extensive areas of land have been cleared, including critical land along the river banks, changing runoff patterns and decimating habitat.
- Chemical use in agriculture has led to high nutrient levels with consequent algal growth and occasional fish kills from pesticides or low oxygen levels.
- As a result, many native fish species, including the iconic Murray cod, have disappeared from many locations, and alien invasive species (including the destructive European Carp) now predominate.
Recently a "Sustainable River Audit" was completed and the results published. There is a lot of work to do:
- River flows in about half the basin's 23 catchments have been sufficiently changed to have a serious impact on ecology. (This is perhaps not as bad as I might have thought.)
- The loss of native fish species and replacement with exotic species has occurred at damaging levels in every one of the catchments, with only 3 categorised as being in moderate condition, and the remaining 20 in poor condition or worse.
- The numbers of macroinvertebrates (basically bugs and grubs) is considered an important ecological measure, because they are a major food source for fish. Their condition more or less reflects that of the fish.
- In all, only one catchment (the Paroo, in the arid far west, and therefore not so heavily "developed") remains in good condition, two other catchments (also in more arid areas) are in moderate condition. The remainder are assessed as in poor or worse condition.
Other factors affect river health: vegetation, channel erosion, location within catchment, climate, etc, and the first two of these will also be measured in future surveys.
There has been much talk about returning water to the river by buying irrigation water from farmers and more efficient water use, but this information suggests that the causes are much more than just over-use of water. Australian governments, national and state, have a record of doing studies and making plans, but generally sitting on their hands. The farming lobby has a lot of influence. We'll see how much is done about this.
Check out the Audit at the Murray-Darling Basin Commission.