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Impounding the Mary River at Traveston Crossing will invariably result in increased degradation of coastal deltas due to a reduction in sediment input. The consequences of reduced sediment also extend to long stretches of coastline where the erosive effect of waves is no longer sustained by sediment inputs from rivers and hence is likely to impact on the Great Sandy Strait and Fraser island by:
· modifying, altering or inhibiting processes (for example, by accelerating or increasing susceptibility to erosion on coastal beaches on the western side of Fraser Island; · modifying or inhibiting ecological processes in a National Heritage place; · reducing the diversity of or modifying the composition of plant and animal species in a National Heritage place due to greenhouse gas emissions that cause global warming.
The Report of the World Commission on Dams (2000) states: “The reduction in sediment and nutrient transport in rivers downstream of dams has impacts on channel, floodplain and coastal delta morphology and causes the loss of aquatic habitat for fish and other species. Changes in river water turbidity may affect biota directly. For example, plankton production is influenced by many variables, including turbidity. If this is reduced due to impoundment, plankton development may be enhanced and may occur in new sections of a river.”
Reduction in sediment moving downstream from a dam leads to degradation of the river channel below the facility. This can lead to the elimination of beaches and backwaters that provided native fish and turtle habitat, and the reduction or elimination of riparian vegetation that provides nutrients and habitat for aquatic and waterfowl species. This is likely to have significant impact on listed threatened species.
Changes to the flow and volume and frequency of sediment load within the Mary River system is also likely to impact on the seagrass beds within the Great Sandy Strait. Migratory marine mammals, Dugong dugon (dugong), Caretta caretta (loggerhead turtle), and Chelonia mydas (green turtle) are all known to feed on these seagrass beds. See Threatened Flora & Fauna.
The impact of spillway flow downstream will scour the river bed and banks for many kilometres. Extensive erosion of downstream beds and banks are often recorded after dam construction due to concentrating flows from the spillway and increased water energy due to loss of sediment deposited upstream of the dam wall. This will have significant impacts on habitat of all aquatic and terrestrial species down stream for an indeterminate distance.
After the construction of Baroon Pocket Dam in the Mary River catchment (a relatively small dam compared to the latest proposal) destabilisation of banks was observed for over 50 kilometres downstream (submissions by the Mary Catchment Protection Group cited in Department of Primary Industries (DPI)1994—per comment John Bradley 2006.)
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The Trapping of Sediments and Nutrients |
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PROPOSED MARY RIVER DAM
IMPACTS ON THE GREAT SANDY
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‘Heavier sediments from the Mary River ... are swept north into Hervey Bay by strong tidal currents. Some larger grained sediments accumulate south of Woody Island, which lies across the northern entrance of the strait. Finer riverine silts and muds drift from the Mary River south to the centre of the strait where the tide causes a large depositing of silt and the greatest accumulation of mud banks, islands and mangroves in the region. Stewart Island (Coonangoor), Dream Island, the Moon Boon Islands, a patchwork of mangrove islands and the huge wetlands complex of Boonlye Point are testimony to the accumulation of silt. (Sinclair, Fraser Island and Cooloola 1991, p.31-32) |
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