Silt socks and silt curtains are a common sight on many Australian worksites, particular during the rainier seasons. Situated properly, they are intended to keep topsoil from running directly off of exposed land and in to the drain. At first look, it is for the protection of the property, and that is certainly true – but the issue of silt in our river systems doubles the stakes.
Runoff and Silt
Almost every river in the world carries a certain amount of silt with it. It gives them their distinctive colour and a piece of their character, and when the ocean slows and reaches the ocean, it gives them their wide, fertile delta. The river’s path through its headwaters, when it is running more quickly, carves out deep valleys, and brings these nutrients with it downstream.
Most organisms and life in a given river have grown accustomed to the visibility and environment. It is when these key attributes are altered that we run the risk of environmental damage.
Case Study: The Murray-Darling System
Australia’s longest river, the Murray-Darling system dominates the watersheds of Victoria, New South Wales, eastern South Australia, and southern Queensland. Heavily exploited, the river is also a classic case of heavy siltification causing damage to species dwelling in its depths.
In this case, siltification is only partly due to construction practices. Invasive carp, brought here from Europe, are bottom feeders, filtering the muck from the river bottom through their mouths and gills for food. Their ability to breed quickly meant that all but the uppermost reaches of the river system are now playing host to these fish, muddying the water year-round.
The impact upon fish species has been profound. The Murray Cod population is perhaps a tenth of what it was historically, as they are primarily sight feeders, and cannot compete in these new, murkier reaches. Australian bass and river blackfish have been similarly affected. Research continues on how best to cope with this threat, but if we can stop this same impact on a smaller scale, we can ideally maintain some habitats in a more pristine state.