Soil erosion is a major issue in Australia , with costs each year in lost agricultural production and consequent degradation of water resources exceeding $500 million . The National Land and Water Resources Audit (NLWRA) [1,3,4] found that sediment loads in many of Australia's rivers exceed the natural loads by 5-20 times. The study also showed that, despite the fact that only ~20% of the sediment transported in the rivers was exported from the river basins, there was significant sediment delivery to many coastal areas. Suspended sediment impacts are of particular concern in wave-dominated estuaries because they have a central basin which forms a sink for fine sediments.
The main processes that contribute sediment to Australian rivers and coastal waterways are gully and stream bank erosion and sheet wash. The NLWRA work showed that each process can dominate in different regions [1,3,4].
Sheetwash and rill erosion occur most frequently in agricultural areas that include annual tillage and seasonally bare ground . Sheetwash erosion is significant when groundcover is reduced to below 70%, and is most pronounced when it drops to below 30% . Sheetwash and rill erosion increase in frequency to the north in Australia, because they are accentuated by intense rainfall .
The most current problems of stream bank/bed erosion occur in river reaches where the riparian vegetation has been removed.
Some impacts of sediment loads on coastal waterways include:
Changes in the following biophysical parameters may indicate that a coastal waterway is receiving excessive sediment loads:
More information on aquatic sediments (changed from natural).
Jon Olley, CSIRO Land and Water