Most of the marine areas of Tin Can Bay, Great Sandy Strait and Hervey Bay formed behind the large coastal dune fields of Fraser Island and Cooloola sand masses when the sea level rose to its present level about 7000 years ago.  The waters of Great Sandy Region are protected from southerlies through to easterlies and from oceanic swells by Fraser Island.  They are also protected by law and regulations as part of the Great Sandy Marine Park

Hervey Bay

Moon Point marks both the northern most point of Great Sandy Strait and westernmost point of the island.  North of Moon Point, Fraser Island is lapped by the calm, clear, azure waters of a magnificent bay named by Captain Cook after Augustus Hervey, 3rd Earl of Bristol known in his time as the English Casanova.  Hervey Bay wouldn’t exist without Fraser Island, and without Fraser Island, Hervey Bay wouldn’t exist either.  Each owes its existence to the other and each protects the other. Fraser Island and Breaksea Spit form the eastern edge of Hervey Bay and protect it from the prevailing southeast winds.  As a result the calm waters inside the bay are a haven attracting small boating enthusiasts.

Hervey Bay is one of the most important estuarine embayments in Australia.  It is one of the largest in Australia and is biologically very rich.  It was only formed when the sea-levels rose to their present level about 6,000 years before present.  Because it is relatively shallow and little circulation Hervey Bay has higher salinity than ordinary sea water.

In Hervey Bay, the sediments are derived from two sources – rivers and the continental shelf.  Sediments from the Mary River are swept north into the bay by strong tidal currents, except for some fine-grained sediments which accumulate south-west of Big Woody Island.  Very fine sand reaching the bay moves shore-wards on to beaches and the remaining sand accumulates offshore.

Coral occurs throughout Hervey Bay with two main areas.  A number of small reefs occur along the northern and eastern margins of Big Woody and Round Islands.

Great Sandy Strait

Great Sandy Strait is a double-ended sand passage estuary and, because of the relatively flat near-shore, has large horizontal tide movements. Low water is one kilometre offshore in some areas. Patterned fens have been recorded along Great Sandy Strait and Cooloola. This type of wetland is the only one of its kind in the world.

The Ramsar listed Great Sandy Strait is a sand passage estuary between the mainland and the World Heritage-listed Fraser Island, and is the least modified of three such passages in Queensland. It is the largest area of tidal swamps within the South East Queensland bioregion, consisting of intertidal sand and mud flats (roughly one-third), extended seagrass beds, mangrove forests, salt flats and saltmarshes, and often contiguous with freshwater Melaleuca wetlands and coastal wallum swamps. The Strait is an exceptionally important feeding ground for migratory shorebirds and important for a wide range of other shorebirds, waterfowl and seabirds, marine fish, crustaceans, oysters, dugong, sea turtles and dolphins

Marine deposits of varying proportions of sand, gravel, and mud form tidal delta banks at either end of Great Sandy Strait and tidal-shaped banks within the strait.  The northern tidal delta is derived from sands of the continental shelf moulded by tidal currents, sediment eroded from the western shores of Fraser Island opposite the Mary River, and sands drifting southward along the shoreline of Fraser Island towards Moon Point.

Ocean water enters Great Sandy Strait via Hervey Bay at its north entrance.  This is 10.5 kilometres.  wide at its northern end and a 1 kilometre wide southern entrance at Inskip Point.  The channel of the strait is relatively deep (generally 15-25 metres) and the main northern channels are generally 10-20 metres deep.

Tides in the area are semi-diurnal with tidal ranges dependant upon position in the bay or strait.  The greatest ranges are experienced in the central strait and the lowest on the ocean beaches and in Wide Bay Harbour.

Waters subject to oceanic influences have a relatively constant salinity of about 35 parts per thousand.  However in the estuarine conditions of the strait the inflow of fresh water from adjacent catchments, particularly that of the Mary River, can cause marked changes in the salinity.

Marine Vegetation

Mangroves are best developed in Great Sandy Strait where they cover about 16,300 ha, mainly in the central section of the strait.  Here they form extensive wetland systems and dominate more than thirty islets and islands.  Mangroves also form islands in the Burrum River and border several other streams entering Hervey Bay such as Theodolite Creek, Eli Creek and Wathumba Creek.  Thirteen mangrove species have been recorded for the area.  The most common species are the river, grey, large-leaved, spurred, milky, myrtle, small-stilted and white flowered black mangroves.

The seagrass beds are probably the largest in area along the eastern seaboard of Queensland and have densities commonly greater than 50% of the seabed.  Two species predominate; Halophila spinulosa and Halophila ovalis.  The Hervey Bay area covered is at least 85,500 hectares and extends to depth of about 22 metres.

The estimated area of seagrass in Great Sandy Strait has increased from 4,800 hectares in 1970 to 12,300 in 1988.  The beds occur in several characteristic communities from Tin Can Bay to Moonboon Islands in central Great Sandy Strait.

Seagrass is most important for the conservation of animals such as dugong and turtles as well as being important nursery areas for fish and prawns.

The marine areas of the Great Sandy Region provide an important recreational and commercial fishery which is being managed to achieve sustainable yields.

Breaksea Spit and Ferguson Spit

Breaksea Spit and Ferguson Spit comprise oceanic quartz sand moulded by longshore drift and tidal currents. Exceptional geomorphic features exist off Breaksea Spit where sand flows down thirteen submarine chasms at the very edge of the continental shelf at the rate of about 500,000 tonnes annually.

Big Woody and Little Woody Islands

Big Woody and Little Woody Islands are composed of sedimentary sandstones, siltstones, mudstones, and shales.  These rocks generally underlie younger marine sediments of the northern part of Great Sandy Strait.

Hervey Bay is one of the major feeding grounds for loggerheads in eastern Australia.  Almost all courtship and mating of loggerhead turtles occurs in Hervey Bay near Rooney’s Point.

The Fens

Behind the mangroves of Great Sandy Strait lie some boggy pock-marked peaty swampy areas with traps for anyone venturing into them.  Every carefully camouflaged hole was a potential pit-trap filled with water waiting to catch any unwary intruder.  In 1996 the view of these treacherous treeless areas mainly located behind the mangroves abruptly changed when international scientists who had attended the huge Brisbane Ramsar (Wetlands of International Significance) Convention recognized the se as patterned fens as they were flying back from the Great Barrier Reef.  With the sun behind the excited peat-land specialists alighted from the planes raving about these same peat-lands people had previously avoided.  They recognized them as fens.  Previously fens had only been associated with tundra and alpine areas.  They have turned out to be very significant features of Fraser Island.  They are the only fens in the world to occur as low altitudes (only metres above sea level) and the only ones in at such low latitudes.  The fens have attracted increasing scientific attention. Core samples from the peat show that they have existed for 35,000 years.  They hold a lot of valuable environmental data in a sort of time capsule that still has to be discovered.