“ deposits of silt and sediment on the
lower floodplain of the Murrah appeared to be at least a couple of hundred
years old until we began to test their composition and age…. What looked like
the accumulation of centuries in the Murrah floodplain turns out to have taken
place since about 1960…. Most dramatic still, nearly a third of the deposit
appears to have been dumped by a single massive flood event, back in 1971.”
Dr Peter Wallbrink: European
impact on landscape ‘immense’–Bega District News 26 Jan 1999.
The
sediment studies in the Murrah catchment, one third cleared and two thirds
forest, point to a hugely increased rate of soil erosion over the past 50
years. This timeframe coincides with the NSW Forestry Commission’s first observations
that forest vigour and growth had declined.
It also coincides
with the Commissions first failed attempt at ‘Timber Stand Improvement’ in
Mumbulla State Forest. Rather than considering that soil fertility had reduced,
the Commission convinced themselves that too many sawlogs had been taken out
and the best way to improve the forest was to cut it all down.
All soils in the Five Forests are erodible and dispersible (Tulau, M. 1997). This fact means that soil loss
and degradation has continued unabated and assisted with logging, burning and
roading.
High intensity rainfall events are generally 10 times the rainfall intensity used to determine erosion estimates (Rosewell and Turner, 1992). Under these conditions every road becomes a creek and every gully a river. This vast road network acts as a huge conduit for sediment across the landscape and into every stream.
Soil dispersion increases in lower layers when forests are logged and burned irrespective of rainfall intensity. Tree mortality during dry weather, drought or as a result of wind-throw will similarly increases the rate of soil erosion and dispersion.
In a drying climate with declining forests and despite all the evidence, management focus on ‘traditional’ logging and burning and still claim these activities ‘maintain and improve’ the environment.
As the soil degradation process continues large trees are replaced with highly flammable scrub. Mass movement of soils on slopes will increase and estuaries will fill with sediment.
During this process surface water will be polluted and in some locations, like the Upper Brogo catchment, this pollution is likely to be a permanent feature. The State government may be planning for this with a new power-line down Brown Mountain. Boosting electricity supply provides the option of installing a desalination plant.