6.14 Erosion and Sediment

Water quality testing shows high turbidity levels after periods of rainfall in both the freshwater catchments and the Middle and Upper Manning Estuary. High turbidity is caused by sediments suspended in the water column. Along with agriculture as previously discussed, activities that contribute to erosion and sedimentation include:

Unsealed roads, stock crossings and unsealed driveways Forestry operations including dirt roads and timber harvest65 Construction sites disturb soil and create dust and debris. Construction includes Council road, bridge and drainage works and private infrastructure such as homes and dams. There is community concern that boatwash is causing significant bank erosion in the Lansdowne River and a study identified the Dawson River was at risk, exacerbated by historic clearing of riparian vegetation (McKeown and Associates, 1997). Flood, tide and wind waves are of more concern in the Manning main channels.

The bushfires of late 2019 burnt 244,173ha, representing 30% of the catchment, with several subcatchments burning over 90%. The potential for soil erosion after a bushfire can be severe due to the destruction of ground cover and the litter layer. The estuary has an average flushing time of 31.6 days, compared with a State-wide median estuary flushing time of 9 days (Roper et al. 2011). Due to the long residence time of fresh water, the estuary is sensitive to the accumulation of sediments.

High concentrations of suspended sediments in rivers can: • diminish light needed for photosynthesis, reducing condition and extent of aquatic plants including seagrass at the base of the food chain • cause eutrophication of rivers and wetlands • make water unsuitable for irrigation and cattle • increased the cost of potable water treatment and infrastructure maintenance • smother the stream bed, macroinvertebrate habitat and seagrass • inhibit respiration and feeding of stream biota.66 Stakeholders: MidCoast Council, Hunter LLS, Transport for NSW, Department of Primary Industries - Fisheries, DPI - Forestry, Mid Coast 2 Tops Landcare, Environmental Protection Authority, recreational boating associations, landholders. Related issues: Loss of riparian vegetation, stormwater, agricultural impact.

65: Midcoast Water, 2011 66: NSW Government, 2009