A tale of two great rivers

Flood irrigation has devastated thousands of hectares of the silt deposited by the river. The crops look healthy, the soil looks dead, an ugly inanimate grey. It is no more than a supporting medium for the crops fed on chemicals. Each summer phosphorus and nitrogen run-off from farms causes a blaze of cyanobacteria and algae over 18,000 square km of the Gulf. Depleted of oxygen, the water becomes as dead as the soil. Fish and shrimps move away, the creatures that stay in place die.

The Gulf and the bayous are the most intensely developed oil and gas fields in the world. The massive extraction is lowering the sea bed. Bayous are being swallowed by the sea, they are drying up on the landward side because controls on the river prevent their flooding. Since work on the river began, half the wetlands, 40 million hectares, have been lost.

Nevertheless the area is still prodigiously productive. During the spring migration it attracts 24 million birds a day. It yields fish, shrimps, oysters and crabs in huge numbers.

At present the Mississippi is trying to take a shorter route to the Gulf through the Atchafalaya River which has already begun to build a new delta. This will strand New Orleans and the Mississippi port that last year became the world’s biggest shipping centre. There are 24 miles (38.5 kilometres) of wharves along both banks. Engineers are strengthening barrages and say that they have the river under control. In 1927 it flooded 130 kilometres wide. The Mississippi, not the engineers, will have the last say.

The source of the Murray River was defined 20 million years ago when a few million years of heavy rain weathered the raw line of mountains pushed up from underneath, then built on by volcanoes. Fingers of rushing water, one of them the Murray channel, carried sediment into the ocean that then covered the Murray Basin.

During the Great Ice Age which began three million years ago, this sea dried up, leaving a deep deposit of salt. The Murray River covered the salt with water, mud and sand, leaving a flattish plain with rivers meandering through it. It did the work that is still being done by the Mississippi. Furthermore there is deep salt under all the northern section of the Murray-Darling Basin which had been covered by another sea. Australia’s salt is under farmland, not safely under the sea like America’s salt.

Our explorers were all looking for grazing land. In 1817 Oxley found the Lachlan River lost in marshes, in 1818 the Macquarie River in even more extensive marshes. Australia’s rivers were no longer making new country, they were looking after the old. They had devised a filtration system that could handle the biggest floods and an even more remarkable system to handle salt.

Lakes, billabongs, lagoons and swamps acted as settling ponds. I have to talk in the past, we have rendered most of them ineffectual. An astonishing number and diversity of plants made up the active filters. They were designed by density, leaf shape, stem length, habit to remove solids of all sizes, even chemicals in solution. Many of these plants disappeared as recently as 15 years ago.

Salt drainage took place through special aquifers which led to creeks and rivers. They were distinct from the aquifers that ran with fresh water. But for the salt drains to be effective it is essential that rivers drop to low levels, not regularly, not even yearly, but periodically for some months. At times of extended low water they ran with a concentration of salt that exceeded any present levels. Since all fish had a marine origin native fish had no trouble. Birds and mammals drank at springs or lakes – there were many. The salt flushed away with the first flood.

Now, the artificial high level of rivers maintained by dams and weirs has stopped the draining of salt. With flood irrigation lifting water tables, the salt has nowhere to go but up to the surface.

The Murray has changed the position of its mouth at least three times in the last 30,000 years, that is to say the entrance and exit to Lake Alexandrina has changed. It was once a wonderfully productive area. Aboriginal Australians treasured Lake Alexandrina for the big Mulloway that they caught in it.

The long, narrow stretch of water known as the Coorong and the many swamps of the south-east of South Australia were a feeding and breeding ground for millions of birds. The swamps were drained, the Coorong is now little more than a storm water channel. As well, barrages completed in 1940 shut Lake Alexandrina off from the Coorong and from seawater inflow. The lake changed from salt water to fresh water, and since the Murray now has one third of its natural flow, the lake does not get much water, it does not harbour many fish. And the natural flow of the Murray with all its tributaries including the Darling is three per cent of the flow of the Mississippi.