Australians may have had a far more catastrophic impact on their landscape than previously suspected, according to fresh scientific evidence now coming to light.
A team from CSIRO Land & Water and the Cooperative Research Centre for Catchment Hydrology has found signs that European settlement unleashed an episode of erosion, sediment deposition and change in river systems orders of magnitude greater than we have assumed to date.
New ways to identify and date flood deposits in river catchments in Eastern Australia are building a picture of a landscape in dramatic transition over years or decades, rather than centuries, say Dr Jon Olley and Dr Peter Wallbrink.
Metres of mud and sand deposited on river floodplains, which the scientists at first guessed to be the result of hundreds or even thousands of years of erosion, are proving to have happened in as few as 30 or 40 years.
“There’s little doubt modern Australians have underestimated the extent of change we have inflicted on our landscape,” says Dr Wallbrink. “In some cases the rates are staggering.”
His research in the catchment of the Murrah river in southern NSW, dominated by dairying and forestry, is throwing the issue under the spotlight.
“Deposits of silt and sediment on the lower floodplain of the Murrah appeared to us to be at least a couple of hundred years old - until we began to test their composition and age.”
It was the atomic bomb that did the trick. Regular atmospheric testing of nuclear weapons, which began in the late 1950s, spread a telltale layer of radioactive Caesium 137 across the globe. That layer now provides a reliable benchmark for soil scientists wanting to date recent layers of sediment.
What looked like the accumulation of centuries in the Murrah floodplain turns out to have taken place since about 1960, Dr Wallbrink says. More dramatic still, nearly a third of the deposit appears to have been dumped by a single massive flood event, back in 1971.
Subsequent tests will reveal whether it was clearing for agriculture at the top of the catchment or forestry operations in the lower catchment which was mainly responsible for the sediment - and the relative contribution of the two.
This understanding will be vital in devising the best strategies for farmers, foresters and land managers to combat future large scale erosion and deposition events and improve water quality and sustainability, says Dr Wallbrink.
“We’re talking about changing the very face of Australia in comparatively few years, so dramatic is the scale of these events,” he says. “The evidence is building that our landscape underwent radical change.”
Dr Jon Olley is pioneering a technique called optically stimulated luminescence to date single grains of quartz sand in a sediment deposit. This technique is unfolding a new chapter in understanding of how we have reshaped the continent.
“Before European settlement, the picture is of a relatively stable landscape, well-vegetated, with lots of swampy meadows in the low lying areas to trap the sediment and nutrients and filter the waters slowly,” he explains.
“The river systems at that time would have been largely clear-flowing, generally slow and dominated by organic material.”
Enter European settlers and the landscape chemistry changes violently. Overclearing and heavy grazing combined with Australia’s regular cycle of drought and flood to unleash a new pattern in the rivers: spates of silt sandblasting the system caused profound changes in the rivers themselves and the life they supported.
“We went, in effect, from slow rivers dominated by organic material to rivers dominated by rushes of abrasive inorganic sediment. This had huge consequences for native fish, animals, water plants and insects.
“Regrettably,” says Dr Olley, “I don’t think the original system is restorable. We can’t put back the clock and have it the way it once was.”
However both scientists consider it likely that a new landscape balance has formed, and that the rate of change is no longer as acute as it was shortly after clearing.
Nevertheless the combination of a cleared landscape with periodic episodes of natural droughts and floods has created a river regime that is now far more energetic and prone to violent flooding than previously existed.
“It’s all about energy,” says Dr Wallbrink. “In the original rivers the rainfall was held back by vegetation and swampy areas. Today it rushes downstream in defined channels far more quickly and in larger volumes.
“It is this new energy which underlies the dramatic rates of change we are starting to see and understand for the first time.”
More information: Julian.Cribb@nap.csiro.au
Materials provided by CSIRO Australia. Note: Content may be edited for style and length.
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