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Geomorphology & Hydrology— The Making of the Great Sandy |
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ourgreatsandy.com Celebrating and Defending the Great Sandy Strait
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PROPOSED MARY RIVER DAM
IMPACTS ON THE GREAT SANDY
ABOUT THE GREAT SANDY |
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’Geomorphologists tell us that 10,000 years ago, when Fraser Island was part of the mainland (like Cooloola is today), Great Sandy Strait was the ancestral bed of the Mary River. They point out that the syncline represented by Woody Island would have deflected its flow to the south so it flowed along what is now Great Sandy Strait. When the sea levels rose over 120 metres about 6,000 years ago, the sediment carried down the Mary River settled in Great Sandy Strait and filled up what would have been a deep river valley there. (FIDO, Moonbi 113, p. 14.) |
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Help Save Fraser Island’s Unique Marine Environment! |
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The Great Sandy Strait is one of the few passage estuarine landscapes in Australia where an offshore barrier island—i.e. Fraser Island—formed sufficiently close to the mainland to block the flow of a substantial river system—i.e. The Mary River—creating a double-ended estuary with a shifting (though relatively stable) pattern of mangroves, sand banks, and mud islands.
This double-ended estuary contains a diversity of marine and coastal wetland types with examples of eight of the eleven marine types. It is the largest and the least disturbed wetland area in southern Queensland. It has large horizontal tide movements because of the relatively flat nearshore. Low water is one kilometre offshore in some areas.
The north entrance of the strait into Hervey Bay is 10.5 km wide; its southern end at Inskip Point is only a kilometre across. Fraser Island provides shelter to the strait’s large system of bays and channels, which are relatively deep (15-25 metres). These waters contain extensive seagrass, mangrove and calm deep offshore areas. The hydrological and nutrient flows between fresh and marine wetlands are highly significant to flora and fauna in the strait and contribute to its numbers and diversity of species.
The Mary River The Great Sandy is mostly a saline water system, but permanent or seasonal flows of fresh water enter most estuaries in the system. The major source of freshwater and sediment flow, the Mary River, discharges 2300 gigalitres of water into the strait each year. Water exchange rates with the open ocean are rapid, owing to the presence of a deep main channel and the absence of mobile, unstable sand bars. In the estuarine conditions of the strait the inflow of fresh water from adjacent catchments can create prolonged conditions of low salinity. This is particularly true of the Mary River mouth. The outflow from the Mary and other major streams has a substantial influence on the water quality of the Great Sandy. Inflows from smaller permanent or seasonal freshwater creeks occur mostly during the wet summer months. Some freshwater is also present in swamps behind mangroves.
Water quality is generally good although inputs of sewage, stormwater, and run-off from agricultural and urban development may cause decline in quality.
Rock, Sand & Islands As the name suggests, intertidal and subtidal sediments in the strait are sandy, but these vary from being sand to mud. The western section of the bay is floored by old sands and gravels deposited during periods of lower sea level. Most of the fine sands along the south-western shore of Hervey Bay are derived from these old sediments by onshore wave transport. Sediments from the Mary River are swept into the south-east of Hervey Bay via the Great Sandy. The finer sediments form mud and sand flats.
The western shore of Hervey Bay is predominantly rocky, the source of these rocks being basalt lava flows from a volcanic eruption of the nearby sloping hummock hills which took place around a million years ago. The rocks only extend about 100 metres sub-tidally, except for Burkitt’s Reef near Bargara, which extends in a north-easterly direction for about one kilometre.
Islands within the strait play a role in the exchange of water, sediments, nutrients and pollutants, between the terrestrial, estuarine and marine environments. Islands such as Big Woody and Little Woody, in the north of the strait, are composed of sedimentary sand stones, silts stones, mud stones, and shales. At the southern end of Great Sandy Strait there are areas of “coffee rock” and harder rock below low water mark, creating scattered reefs (G. Brooks pers. comm.).
Information courtesy of the Department of Environment and Heritage. P.O. Box 155 Brisbane, Albert Street QLD, 4002.
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The Mary River mouth with Fraser Island in the distance. |
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© StewArt Photography |
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Website created by Elisabeth Berry for 2Berries Communications. Copyright © 2008 ourgreatsandy.com. All rights reserved. |
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Little Woody Island (left) and Picnic Island. Fraser in the distance. |