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.