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The Basin - Present, Future and Past

The Basin; a Coastal Lake

St Georges Basin (the lake)

There are many coastal lakes in NSW and St Georges Basin ("the Basin") is just one of these. Most are a result of (1) sea-level rise at the end of the Quaternary (for an explanation of this term see the glossary at the end of the Geology section), (2) wave and tidal action, and (3) erosion in the catchment areas. Many are relatively shallow and have narrow connections with the sea which are occasionally closed by sand driven by off-shore currents. A large barrier of sand exists to the southeast of the Basin behind Bherwerre Beach restricting the ocean connection to the narrow Sussex Inlet.

The Last Glacial Maximum (see the glossary) was some 19,000 years ago. The amount of water tied up in continental ice was such that the sea-level all over the world was about 120 m below the present level. At this time Australia and Papua New Guinea were connected by a land link and locally the ocean was about 15 km out from Bherwerre Beach. When the ice melted the sea-level rose quickly (by geological standards) - about 110 m in 10,000 years. After this the change was slower and by about 7000 to 6000 years ago sea-level was only about 2 m below the present level. Prior to this Wandandian Creek would have flowed directly out to sea - probably on a line through the present Bherwerre Beach. It would have had tributaries such as Tallarwalla and Tomerong Creeks. Our present coast line was shaped in the remaining 6000 years.

The lakes provide habitats for various marine and terrestrial flora and fauna. For example, coastal lakes are the source of food and shelter for many types of birds - such as pelicans, herons, ducks, ibis, kingfishers, cormorants, sea eagles, etc. The problem is that human beings also like to live and work near lakes (and other sources of water). Development pressures impact on the lake, denigrate the water quality and affect the ecosystems.

The salinity and water quality depend on the circumstances of each lake; they can differ from lake to lake, and from place to place in a particular lake depending on its geometry. They can also depend on the amount and quality of water fed into the lake by creeks and rivers, by storm water from developed areas, and on the condition of the channel. Many have extensive areas of seagrass which, like mangroves, are favoured havens for juvenile fish. It is worth noting that the Basin contains a considerable proportion of the total seagrass area of the NSW South Coast. In general the marine fauna depends on the nature of the lake but certainly many lakes are nurseries for young fish. The Basin has been fished by professionals and amateurs alike ever since Europeans came to settle in the area, and no doubt by the indigenous Aboriginal people long before that. Commercial fishing in the Basin ceased at the end of 2002.

The Basin has an area of about 40 square kilometers and a maximum depth of 9 metres; its catchment is about 390 square kilometers. Connection with the sea is via Sussex Inlet which is about 6 km in length. The major tributary of the Basin catchment is Wandandian Creek which starts in the Morton National Park up on the escarpment plateau to the south west. Development (suburbia) stretches along the northern edge of the Basin from Wrights Beach to Basin View, and along the western bank of Sussex Inlet. The remainder is relatively unspoilt and supports areas of casuarina, river mangrove, saltmarsh and sedge. The Corramy State Recreation Area adjoining Basin View has recently been acquired by the National Parks & Wildlife Service and a Plan of Management has been prepared. A separate portion of Corramy west of Tullawalla Lagoon has become part of Conjola National Park.

In recent years the NSW state government, through the Healthy Rivers Commission (HRC), instigated a review of the coastal lakes. This Independent Inquiry into Coastal Lakes was open for public for comment and sought to find improved strategies for managing coastal lakes. As part of the Inquiry lakes, were classified according to certain environmental factors. The final report came out in June 2002. The Basin was placed into Healthy Modified Conditions, a category which allows further development. This assignment was listed as provisional requiring urgent further assessment. In our opinion it would have been preferable to have the Basin in the Significant Protection category which would have restricted further development to within existing developed areas.

In February 2003 a "Statement of Intent for the Coastal Lakes of New South Wales" was issued by the NSW State Government. This statement is the Government's response to the HRC Final Report of June 2002 and records its commitments, the specific actions to be implemented, and the time scales for these. Unfortunately the Basin was not placed on the priority list so developments will continue for at least 3 years before the lake receives a sustainability assessment.

A recent survey (November 2003) by Geoscience Australia has found that the nutrient levels are high; the Basin is in danger of becoming eutrophic - where lower layers become short of oxygen, and upper layers produce excessive organic growth such as algal and phytoplankton blooms. This on-going process is reducing the area covered by sea grasses. The survey also noted that St Georges Basin had more ammonium available for plant growth than almost all other temperate coastal lakes/estuaries in Australia. The Basin has another problem - an infestation of the exotic marine weed caulerpa taxifolia in a 6.5 hectare region beside Basin View. As in other infected coastal waters NSW Fisheries are attempting to eradicate the caulerpa using large quantities of common salt.

This section has water at its core. Now water is essential to life on this planet and has some truly amazing properties. With this in mind a supplementary page has been created to inform the reader on some of the properties of "amazing water". Just follow the link.

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Global Climate Change

St Georges Basin as we know it is a relatively recent feature of the South Coast - relative, that is, to the time scales associated with glacial cycles. The Basin was formed over a period of perhaps 6,000 years. Sand drifting across an open bay formed by the rising ocean produced the lake we now call St Georges Basin. The sand barrier is known as the Bherwerre Barrier.

We are at present in an interglacial period (see the glossary) and probably could expect, in time, to move towards another ice age. Just how long this might take is completely unknown. However, another aspect, namely anthropogenic (human induced) global warming now needs to be considered. Due to human activity the concentrations of the so-called "greenhouse" gases such as carbon dioxide and methane are rapidly rising in our atmosphere and are considered (by many) to be responsible for an increase in the planet's temperature.

It is interesting to note that these gases are present only in trace amounts. For example, the carbon dioxide concentration is only 0.04% (actually 380 parts per million by volume).How can these trace amounts produce a significant effect? The answer lies in the presence of water vapour. This is the important greenhouse gas and responsible for the planet's temperature being some 30 Centigrade degrees higher than if it were not present. The small warming effect of carbon dioxide, methane, etc acting on the oceans produces a positive feedback effect; an increase in the water vapour concentration which in turn produces an amplification of the temperature increase. Evidence of this effect (the increase in water vapour concentration) has recently been instrumentally observed.

An increase in the average temperature of the planet will produce many undesirable effects, some of which may be already evident. In Australia we can expect more droughts, more days of high temperatures (e.g. above 35 degrees Centigrade), more intense rainfall events, coral bleaching, etc. A small rise in sea-level is already evident. Most of this increase, which is presently about 3 mm per annum, is simply due to the oceans getting warmer and expanding. However, there is a contribution (thought to be less than 0.1 mm) due to the melting of ice-sheets, glaciers, etc. If the global temperature continues to climb we can expect the contribution from ice-sheets to become very significant.

10 m sea level rise

It is possible to measure the amount of ice held in the planet's major ice-sheets (Greenland and Antarctica). If the Greenland ice-sheet completely melted it would raise sea-level by about 7 m. If the Antarctic ice-sheet were to melt the rise would be about 70 m! A sea-level rise of some metres would obviously have deleterious effect all over the planet because many of our major cities are on the coast. What would be the effect be in the St Georges Basin area? The diagram here shows the change in the local coast-line should the sea-level rise by 10 m. This is a convenient value to use because the 10 m contour is shown on topographic maps (1:25,000).

We see that the Bherwerre Barrier is removed and St Georges Basin again becomes a wide open bay. The Bherwerre Pensinsula would become an island if, as is quite likely, the sea broke through at the point marked with the red cross. This is likely because a characteristic of global warming is an increase, in both intensity and frequency, of storm surges. Many local townships are covered in whole or in part; Huskisson, Vincentia, St Georges Basin, Sanctuary Point, Basin View, Sussex Inlet, Cudmirrah to name a few.

However, please note that a 10 m rise may never occur if we can control and reduce our greenhouse gas emissions. And even if the worst scenario occurred it could take 100s of years for this to happen. Governments around the world are slowly moving to reduce greenhouse gas emissions. This will require the world to reduce its dependence on the traditional sources of energy, i.e. coal and oil. Australia is one of the worst emitters of greenhouse gases (on a per capita basis).

It should be pointed out that not everybody believes in the existence of anthropogenic global warming. However, in the general public and in the scientific community such persons appear to be in the minority. A good example of "non-believers" are geologists whose thesis is that our planet throughout its life has experienced many episodes of climate change and that what is presently occurring is not at all unusual. Furthermore, they tell us that in the past the carbon dioxide content of the planet's atmosphere has been significantly higher than it is today.

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Geology

This section attempts to provide a brief introduction to the geology of the St Georges Basin area. A glossary is provided to explain the meaning of certain terms. One can define the geology of an area to be the nature of its bedrock. But we should remember that the nature of this bedrock and its interaction over millennia with its environment, determine the present topography, affect the nature of the overlying soils, vegetation and wildlife, and the usefulness of the area for human activity. So the Basin we see today is the result of many factors, often interrelated, one of which is the local geology.

To begin with it should be pointed out that the Basin area is part of a much larger geological structure known as the Sydney Basin which, approximately, stretches from the Hunter Valley to Batemans Bay along the NSW east coast and westward to beyond Lithgow. A simplified geological map of the St Georges Basin area is shown below. It is based on the appropriate part of the Australia 1:250,000 Geological Series Ulladulla Sheet. (ref 1). A look at this tells us that the bedrocks (mostly sandstones) belong to the Permian period; a geological period occurring between 230 and 280 Mya (see the glossary for an explanation of these units) when Australia was close to Antarctica. The overlying soils tend to be thin and poor in quality; layers of clay are often present. In some areas considerable Quaternary deposits overlie the Permian bedrocks. Creeks entering the Basin, Wandandian, Tullawalla, and Tomerong for example, flow through Quaternary deposits (alluvial gravels, sand and swamp deposits) in their lower reaches. And on the ocean side between the Basin and Bherwerre Beach there are enormous Quaternary deposits - mainly in the form of old sand dunes. There is a small area of Tertiary gravels just south of Vincentia.

strata definitions geology map

The bedrocks around the Basin are members of a set of horizontally bedded Permian sediments known as the Shoalhaven Group. These rocks, sandstones, siltstones and conglomerates, were created in a polar-marine environment. To the north of the Basin the bedrock is Wandrawandian Siltstone, to the east is the Snapper Point Formation (a sandstone which was previously known as, and which is shown on the Geology map as, the Conjola Formation) while to the west both are present. A bit further away we can find the same rocks in Pigeon House Mountain. Here, the Wandrawandian Siltstone, the younger of the two, can be seen above the harder sandstone of the Snapper Point Formation which forms the lower cliff line. The rocky top of Pigeon House belongs to another (even younger) member of the Shoalhaven Group, namely Nowra Sandstone. A recent local geological map (ref 2) is centred on Jervis Bay but covers the eastern and southern regions around St Georges Basin. The Snapper Point sandstone and Wandrawandian siltstone are shown on this map with slightly amended boundaries.

A variety of fossils can be found in the Permian bedrocks of the southern Sydney Basin; for example, bivalves, brachiopods, gastropods, bryozoans, and crinoids. Glendonites are an unusual "fossil" and can be found in the area. To learn about glendonites follow this link. If you have an interest in fossils, especially those found on South Coast rock platforms, then take a visit to Ulladulla. Here there is a project known as the Gondwana Coast Fossil Walk. There is a small museum with many fossils to peruse. Glendonites are included in the display. Furthermore you can take a guided walk out onto the rock platform. Contact the Ulladulla Visitors Centre for more information.

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Glossary

Units of time: time in years before present is here given as ya (e.g. 5 million years ago is 5 Mya). It should be noted that geological ages depend a little on the information source inspected. The terminology in use here has M representing a million and k representing a thousand.

Permian: A geological period between 300 Mya and 250 Mya. Sometimes called the age of amphibians; a time of amphibians and reptiles on land and many marine species. Australia was attached or close to Antarctica at this time. The period ended with a mass extinction of life on Earth created by huge volcanic eruptions. It is estimated that 96% of marine life, and 70% of terrestrial life, disappeared. This is the largest extinction in the history of life on our planet

Tertiary: This is the period from 65 Mya to 1.8 Mya. Sometimes called the age of mammals. At the end of the preceding period (the Cretaceous; the age of dinosaurs) there was a very significant global extinction event initiated by the impact of a meteorite. On land this led to mammals taking over from dinosaurs in the Tertiary. Among the earlier mammals were primitive primates - from which the first hominids much later evolved. By this time the locations of the continents were much as they are now.

Quaternary: This is a term describing the most recent period in the Earth's history - from about 1.8 Mya to the present. It is usually subdivided into the Pleistocene (from 1.8 Mya to 10 kya). and the Holocene (from 10 kya to the present). Our immediate human ancestor, homo sapiens, evolved in the Pleistocene. There were several glacial cycles during the Pleistocene. The last ice age also began and finished inside this period.

Last Glacial Maximum: This is the time in the last ice age when the continental ice sheets had their maximum extent. It occurred about 19 kya

Interglacial: An interval of warmer global average temperature that separates ice ages.


References

  1. Australia 1:250,000 Geological Series Sheet S1 56-13 (Ulladulla) 1966
  2. Kowari 5: Jervis Bay a Place of Cultural, Scientific, and Educational Value edited by G. Cho, A. Georges, R. Stoutjesdijk and R. Longmore, and published by the Australian Nature Conservation Agency, 1995

Notes

  1. The geological sheets published in 1966 are the most recent available. Surely it is time for a 2nd edition!
  2. There are more recent geological maps available but they cover limited local areas. That found in reference 2 does not cover the whole of the St Georges Basin area.
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