|Aerial view of the Murray River. Photo: Simon O'Dwyer, The Age, 2008|
Meanders are part of the Murray. They are as typical of low gradient rivers as braiding is in faster flowing ones. They occur because rivers like the Murray erode on the outside of their bends and deposit on the inside. This simple mechanical process contorts the river from one side of its valley floor to the other. Given enough time, every part of the valley floor will one day have had the river pass through it. This movement of the river throughout its valley is what creates the broad flat planes, so often falsely attributed to flood events.
Meandering rivers create beaches and sweeping bends. They create short-cuts (called cuttings) when meanders move too close and tall cliffs when they butt up against hard rock in the Lower Murray. Cuttings are fast flowing and full of snags when they are young. They should be approached with caution. When in doubt, go around. Over many decades the cuttings establish themselves as the preferred path of the river. The old meander becomes isolated and slowly fills with sediment. These isolated stretches of river bed are called billabongs in Australia. When seen from the air, the Murray is skirted by billabongs, like speed wobbles in a cartoonist's drawings.
The older the billabong, the shallower it becomes. As the former river channel becomes shallower and shallower, they become swamps and eventually gullies. Long lived River Red Gums line the remnant banks hundreds of years after the river has moved on, making this very confusing country to bush walk through. This mosaic of developmental habitats and the rich soil within them drives biodiversity. It is no surprise that it is home to so many species of fish, water birds, turtles and mammals.
"Once a Jolly Swag Man camped by a billabong, under the shade of a coolibah tree..." Is the opening line of Walzing Matilda, the de facto Australian Anthem. It was written by Banjo Paterson in 1895, six years before Australia became a nation in 1901.
Sometimes what might be shown as a cutting on a map looks as if it always was the course of the river: others are snag filled sieves. They can save many kilometres of paddling, be fruitless dead ends, or shallow, reef filled streams. Near weirs their exits can be blocked by four metre high reeds. They represent an unknown, younger river, hiding within the old and in every case are an adventure.
|Detail of a cutting: Murray River Charts: Maureen Wright http://www.rivermurraycharts.com.au/|
When the river floods it connects with its old self again, filling the billabongs with fresh water. Animals trapped in them since the last floods can escape and new ones enter. Eggs from freshwater invertebrates and fish that have lain dormant in the cracked mud hatch. Frogs, such as the pobbelbonk, who have cocooned themselves to survive the drying out of their home emerge with surprising rapidity. The billabongs become havens of life, attracting water birds, pleased to breed in place with such a good food supply.
|John Watson: Understanding the basin and its dynamics.|
|Details from Stage 2 of the MDB Plan: focusing on interconnectedness.|
Since the settlement and intensive use of water in the Murray Darling Basin by our growing population, the floods which naturally recharge billabongs happen less frequently. This is because the same amount of water has to not only keep an environment which has grown to depend on it, but also the drinking, living, irrigation and industrial needs of the 3 million people that live along the Murray and its tributaries, plus the many more who depend on the food produced there. Small communities, like Lake Boga all but died as their lakes dried up. River Red Gum forests which need seasonal flooding to remain healthy have also suffered. Sawmills all along the river closed. To alleviate this problem, networks of channels have been built, and, in recognition of the importance of a healthy ecosystems to a healthy environment, natural assets, like the lakes, billabongs and forests now have water allocations managed as part of the catchment-wide irrigation budget. This means, that the forest and its billabongs can be irrigated without flooding the river. More from less. It is a step in the right direction.
|Map showing how the Gunbower Perricoota Koondrook Forest can now be flooded without inundating the surrounding land.|
|Regulators hold back water until it is ready to be released back into the Murray. 70% of water used returns to the Murray River.|
|Levees divide the forest into sections, much like an irrigated paddock.|
The difference between the way a braided stream erodes and a meandering river has to do with the force the water is flowing. Braided streams have enough energy to erode on both sides. They can push their way past hindernesses. In the Upper Murray and Campaspe Rivers, willows have been removed because the rivers are widening, eroding farmers' land a creating shallower rivers, as their beds fill with sediment. The loss of habitat diversity leads to a loss of species diversity. Animals which like to live in deep holes in the river, such as Murray Cod and Murray Crays leave the area.
|A braided river. When a river shifts its bed it is called an avulsion. For more information about avulsions: read http://www.geo.uu.nl/fg/palaeogeography/results/avulsions|
Curiously, when maps (or satellite images) of the river in the Upper Murray, Barmah Forest and Gunbower Perricoota Koondrook Forest are observed at such a large scale that the meanders appear almost as straight lines, then the many anabranches in those areas give the river a braided appearance. It seems that one form of river movement over time does not exclude the other.
|Meandering Murray downstream from Albury.|
|Multiple anabranches near the Barmah Millewa Forest give the river a braided appearance when observed on a macro scale.|
These animations show how rivers move over time. Although it is difficult to imagine things that take longer than the human lifespan to happen, the signs that they do are everywhere, and the process of erosion and deposition that drive them, visible on a daily basis.