A tale of two great rivers

I’ll tell you a story about two rivers, the Murray and the Mississippi. Mississippi – the word brings thoughts of Ol’ Man River. But the Murray is the old man, the Mississippi is young. When thousands of metres of ice slid over America’s ancient mountains three million years ago, it so tore them about that the present great Mississippi valley was not formed until the ice began to melt only fifteen thousand years ago.

Big floodplain rivers are highly productive. The upper and middle reaches of the Mississippi and its tributaries were alternating areas of prairie, even more open than Australia’s grassy woodlands, and forested wetlands. Species richness was high, so was the diversity of age groups.

Each river is its own marvel. The Mississippi is especially marvellous because it is so rampantly wayward. It produces whirlpools a hundred metres across, spinning so quickly they look like patches of dead calm water until you run into one in a boat.

Because much of its water comes off the Rocky Mountains which are 4000 metres high, the Mississippi is a fast river, flood currents reach eighteen knots. The river tears islands to pieces and uses them to build thousands of islands lower down. The river has taken whole towns and run a new course over the site where the town was.

As with the Murray-Darling basin, the level of the rivers drops each summer giving respite to those plants that did not appreciate constant water about their roots.

Since the Mississippi began to flow it has spread rich soil 96 metres deep across the plains and extended the coastline 130 km by 230 km wide into the Gulf of Mexico. In doing that work it has made six shifts of its mouth.

That area carries the lovely Choctaw Indian name bayous. It is a bewildering assemblage of rivers, bays, lakes, sounds, marshes and swamps. Strange things happen there too. As the racing river drops its mud in the Gulf, the force of the new mud striking previous deposits forces them up until they rise as islands about a metre high. These islands can rise suddenly enough to lift a ship.

There are thousands of salt domes on the Gulf coast. When the sea evaporated 180 million years ago, it left thick deposits of salt. Rising seas covered the salt with hundreds of metres of sediment whose weight pushes down on the lighter salt and forces it to the surface in huge domes.

The river system has changed and is still changing rapidly, due partly to the vagaries of the river but mostly to the vagaries of engineers who set out to improve the river as soon as people settled on it and built boats.

Like all rivers the Mississippi creates natural levees along its banks. When a river overflows its banks, it deposits the heaviest and bulkiest sediment first, then drops silt in decreasing amounts as it spreads. The result is a slope up to the river. Parts of New Orleans are seven metres below sea level. By 1726 those in charge of the town had ringed it with an artificial levee 1.5 metres high. Plantation owners built levees to protect their crops. A levee on one side of the river necessitated a levee on the other side. In the early 1800s engineers began what they called ‘improvements’ to the river. By 1858 there were more than 1700 kilometres of levees.

Snagging was the next operation, the removal of ‘the teeth of the river’ that would savage a boat. Engineers dug through sand bars, removed rocks and cut passages through shoals. As the numbers of big steamboats increased, United States Army Corps Engineers dredged stretches of the river to increase depth. They built hundreds of what they called wing dams to slow the current near the banks and divert the fast current towards the centre. They dug cutoffs to shorten the river, they dug channels as flood relief.

Neither river nor land appreciates the work of the engineers. High water levels are causing a change in the forests. Inferior species like willows, Silver Ash and Pin Oak are taking over country that no longer gets flooded. There is much less diversity.

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.

Engineers have worked the Murray with the same destructive effect as the Americans. In the 1850s snagging parties rowed up from Wellington in South Australia to clear the river for the new paddlewheel steamers. Huge Murray Cod had their homes of a hundred years sawn up and dragged out of the river by horse teams. So much life was pulled out with the logs that the South Australian section of the Murray lost many of its invertebrates.

Animal and vegetable life in rivers depend on woody debris for food and shelter. When a big River Red Gum falls into a river it can dump more than 8000 invertebrates into the water, spiders, bugs, beetles, flies, ants, wasps, scorpions, moths, others in lesser numbers. They are immediate food. Bacteria, fungi and invertebrates then begin work on the leaves and wood providing a feast of organic matter for hundreds of creatures visible and invisible.

The boilers of the paddlewheelers required wood to fire them. An eight-week trip used about 140 tonnes. Since for some 30 busy years a hundred or so steamers plied 6500 km of the Murray-Darling rivers, enormous quantities of timber were burned to power them.

As river traffic gave way to railways, irrigation began. From Blanchetown to the border in South Australia and along the Murray to Mildura and beyond, scores of steam engines set down on the banks to drive big centrifugal pumps with long, crossed, leather belts. Each engine used more than 30 tonnes of wood a day.

All the water pumped was for flood irrigation. Despite the fact that Australian soil began to protest as early as the 1920s that it could not tolerate that method of watering, the Snowy Mountains Hydro-Electric Scheme was completed in 1972, a social and engineering marvel, an environmental disaster. It harvested two beautiful rivers, the Snowy and the Eucumbene, and turned their water into the Murray and the Murrumbidgee to salt thousands of hectares of river flats by unsustainable methods of irrigation. The recently announced increased flow to the Snowy will not change methods of irrigation. The extra water will come from improved methods of delivery to the farmers who are misusing water.

Sixteen dams were constructed as part of the Snowy work. Other dams on other rivers followed, the most stupid of all the Copeton dam on the Gwydir River. Before that dam some of the world's richest land, the Moree watercourse country, was laid out naturally to be flooded every few years. In a high flood an extent of country 80km long and 80km broad received gentle flooding varying in depth from a few centimetres to a metre. The growth of pasture after such a flood was prodigious.

Dams and weirs are an offence to native fish accustomed to ranging hundreds of kilometres. Even when fish ladders were put in they were built to a Canadian design for salmon. The engineers forgot to teach Murray Cod and Yellowbelly to jump. All water is released from the bottom of the dams. Such water has little oxygen, it is ten degrees Celsius colder than river water and the temperature holds for 300km, putting native fish off feeding and breeding. If top water only were taken, not a difficult engineering feat, dams would not be nearly so destructive.

It is fair to say that irrigation in Australia has been a failure. Not all the electricity, not all the vegetables, not all the grapes, not all the rice, not all the cotton, can compensate for the damage already done by flood irrigation. If methods do not improve, if all flood irrigation does not stop, 4 million hectares will be salted in the Murray-Darling Basin. That is sufficient land to grow one quarter of Australia's wheat and barley.

Irrigation is essential, we can not produce enough food without it. But it has to be drip irrigation or well-managed sprays. Not a drop more must go onto the ground than the crop can use.

Temporarily, because of salt interception schemes and the calculated release of flushing water from the Menindee Lakes and Lake Victoria, the quality of Murray water is improving, though the Darling and its tributaries are rapidly deteriorating. The Namoi is in special trouble. While the salty watertable is rising on the Liverpool Plains, so much water is being drawn out of deep freshwater bores to supplement the river water that trees are dying. The water has receded below their roots.

There are government attempts, still too weak, to reduce the water taken from rivers and bores. In numbers of areas in the Basin, State Forests are carrying out trials with trees, principally River Red Gum (Eucalyptus camaldulensis), to lower watertables while producing timber and charcoal. Water Resources are also working to maintain River Red Gum forests on the Murray.

Most importantly, it has been realised that lakes should be allowed to function naturally. In Australia a full lake is not a natural lake. In March 1998 Moira Lake was drained, a big lake on the outskirts of the Barmah-Millewah Forest. In 1855 a commercial fishery was established there, taking 150 tonnes of Murray Cod annually. Sometimes the lake dried up, to be stocked naturally with fish again on the next Murray flood. When the lake was held full of water after the dams went in, European Carp took over. Hundreds of thousands, many a metre long, died when the lake was drained. Grass grew in the bed for a few months, then the lake was filled again and native fish introduced.

Like the American Indians, Aboriginal Australians demonstrated that humans can live on a river without destroying it. Before smallpox, the Murray population was high, many thousands. There is hope for the Murray-Darling Basin under our management but it will require years of determined work and courageous government leadership to rectify the damage.