Fine sediment loads

Soil erosion is a major issue in Australia [1], with costs each year in lost agricultural production and consequent degradation of water resources exceeding $500 million [2]. 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.

Human activities that give rise to enhanced sediment loads

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

Sheetwash and rill erosion occur most frequently in agricultural areas that include annual tillage and seasonally bare ground [1]. Sheetwash erosion is significant when groundcover is reduced to below 70%, and is most pronounced when it drops to below 30% [1]. Sheetwash and rill erosion increase in frequency to the north in Australia, because they are accentuated by intense rainfall [1].

Gully erosion

Gullies erode (gully erosion)soils that accumulate in valleys from soil creep and sheetwash erosion, once the protective vegetation has been removed.

Streambed/bank erosion

The most current problems of stream bank/bed erosion occur in river reaches where the riparian vegetation has been removed.

Impacts of fine sediment loads on coastal waterways

Some impacts of sediment loads on coastal waterways include:

enhanced sedimentation rates potentially leading to the shallowing of estuaries [5];
increased turbidity levels, limiting light penetration and photosynthesis;
increased loads of particle-bound toxicants and nutrients (mainly phosphorus, but also potentially ammonium), potentially leading to eutrophication;
changes in the size and distribution of habitats such as mangroves, salt marshes and seagrass meadows;
increased organic matter degradation by anoxic processes (e.g. sulfate reduction);
impacts on benthic invertebrate and fish assemblages caused by the smothering of habitat, the clogging of gills and reduced feeding efficiency and food quality; and
an overall reduction in biodiversity, health and integrity.

Relevant indicators

Changes in the following biophysical parameters may indicate that a coastal waterway is receiving excessive sediment loads:

increased sedimentation rates;
turbidity levels in excess of ANZECC/ARMCANZ guidelines [6];
total phosphorus concentrations (water column) in excess of ANZECC/ARMCANZ guidelines [6];
expansion of mangroves into saltmarsh areas;
a lowering of denitrification efficiencies;
a lowering of TOC:TS ratios;
a reduction in seagrass areas; and
reductions in the abundance and diversity of benthic invertebrate communities and fish assemblages.

Low concentrations of carbon and nitrogen in sediment are sometimes also indicative of the dilution of organic matter by mineral sediment (e.g. causing lowered organic:inorganic sediment ratios) [7].

more information on aquatic sediments (changed from natural).

References

Marston, F. Prosser, I., Hughes, A., Lu, H., and Stevenson, J. 2001. Waterborne erosion - an Australian Story, CSIRO Land and Water, Canberra, Technical Report 17/01.

http://www.clw.csiro.au/publications/technical2001/tr17-01.pdf

Scott, A. and Olley, J. 2003. Settlement, erosion and muddy waters: Lessons from the past.

http://www.mdbc.gov.au/naturalresources/env_issues/muddy_waters.pdf

Prosser, I., Rustomji, P., Young, B. Moran, C. and Hughes, A. Constructing river basin sediment budgets for the National Land and Water Resources Audit. CSIRO Land and Water, Canberra, Technical Report 15/01, July 2001. (http://www.clw.csiro.au/publications/technical2001/tr15-01.pdf)
Prosser, I. and Norris, R. 2001. Muddy waters and sand slugs: sediment transport in Australian rivers. RipRap, Land & Water Australia, Edition 20, pp. 13-16.
see Brooke, B. 2002. The role of sedimentological information in estuary management, Proceedings of Coast to Coast 2002 - "Source to Sea", Tweed Heads, pp. 31-34 (and references).
ANZECC/ARMCANZ (October 2000) Australian and New Zealand Guidelines for Fresh and Marine Water Quality. (www.ea.gov.au/water/quality/nwqms/#quality)
Radke, L.C. 2002. Catchment clearing impacts on estuaries. AUSGEO News 65, 6-7.

Contributors

Jon Olley, CSIRO Land and Water