Fraser Island contains more than half of the world’s known perched dune lakes. These are lakes resting in impervious organically bonded sand-rock in wind-formed dune depressions. In their mode of origin, they are without parallel elsewhere in the world. The largest lakes of this special type are found on Fraser Island. The Great Sandy Region may be termed a “lake district” equivalent to the Tasmanian and Victorian Lake districts.
The freshwater lakes are highly oligotrophic, containing very small quantities of nutrients and are well oxygenated, but of low biological productivity. Such lakes are far less numerous elsewhere in Australia, even in the world.
Wabby Lakes on Fraser Island are examples of barrage dune lakes, not known elsewhere in the world. These lakes have been formed by sandblows damming a natural watercourse.
In terms of chemical quality, the Great Sandy Region has the freshest, naturally running water in the world.
The high dunes of the sandmasses form an unconfined aquifer. The water of the aquifer has the same ionic ratio as sea water. On the eastern coast, the water flows as springs from the aquifer across the beach sands in shallow streams, emerges in springs from the beach, or flows through the beach below sea level.
On the west coast and in some creek valleys the water seeping from the high dunes becomes colonized by peat forming Empodisma and once the peat forms it can establish fens.
Water dating by the CSIRO Division of Soils shows that much of the water emerging in springs on Teewah and Cooloola Beaches has been in the sand for decades. Tritium values show that rain falling in the middle of the sandmasses may take 70 to 100 years to issue as white water along the beaches, whereas the black waters issuing along the western margin are much younger. The close proximity of black and white waters issuing from a single source is of scientific importance.
On Fraser Island there are many streams fed directly by the water table. Eli Creek (80 ML/day), is the largest creek, flowing to the east. Most of Fraser Island is drained to the west by large creeks such as Bogimbah Creek (150 ML/day). The volume of flow in all of these streams is relatively unaffected by seasonal fluctuations in rainfall, since the sandmass acts as a major aquifer releasing its flow at a relatively constant rate. The full extent of the freshwater resources of Fraser Island has yet to be fully assessed.
Both ‘black’ (organic stained) and ‘white’ (colourless) waters occur in the sandmasses as dune lakes or issue as springs and streams draining the region. Despite their frequent proximity to each other, they have different compositions related to the different pedological processes forming podzols and humus podzols.
Brown or tea-coloured, organic-stained water occurs at springs, seeps and perched lakes in the region. Springs of clear, colourless water (‘white’ water) sometimes occur in close proximity to the organic stained water. In the Great Sandy Region both ‘black’ and ‘white’ waters are associated with the podzolization processes. ‘Black’ water gathers its organic tea colour as it slowly drains across the soil mantle or through peaty layers. ‘White’ water is the deep groundwater, which has had the organic stain filtered from it by its passage through deep humate and iron-stained sand deposits. In contrast, many overseas locations have, ‘white’ water which is associated with lateritization and ‘black’ water associated with podzols.
Different types of dune lakes are a feature of the sandmasses. These include a large number of perched lakes, high in the dunes, where surface water is retained above organic pans. “Window” lakes occur where the ground surface sinks below the main white water mound. In the case of “barrage lakes”, young sand dunes have blocked surface streams damming them up to form a natural lake.
Contrary to what one would expect of land masses composed of a material as porous as sand, the Great Sandy Region is well endowed with freshwater lakes. There are 34 named freshwater lakes in the region. Within the whole of Australia, the Great Sandy Region is one of the few areas outside the Central Plateau of Tasmania and the western district of Victoria that can be truly termed “a lake district”.
There are three main types:
- Perched lakes occur in topographic depressions where it is believed that the products of sand weathering and organic matter have accumulated to form semi-impermeable discontinuous layers, or where impermeable layers of ironstone or compressed peat lie close to the surface. The perched lakes usually contain ‘black’ waters unless there is an opportunity for chemical precipitation of the organic colloids, as occurs at Lake McKenzie.
- Barrage lakes occur where mobile sand dunes have blocked or dammed water courses. Barrage lakes may be either ‘black’ and ‘white’ water or be a mixture of both.
- Window lakes occur where the regional water table has intersected depressions in the land surface. Water in window lakes is normally ‘white’ unless coloured by some surface run-off.
Foredunes are developed at the shores of many lakes among the dunes of Fraser Island. They are very well preserved, although they are very old and the shape has been partly altered by prehistoric blowouts.
Although lakes occur in dunes in many areas of the world, the mode of origin of the lakes of the Great Sandy Region, through a combination of wind action and organic accumulation, is peculiar to eastern Australia. Small dune lakes occur in the east coast of New South Wales, Victoria, and far north Queensland. The process may rightly be claimed to be unique.
Most of the lakes in the Great Sandy Region are believed to be perched because they are situated among leached dunes of quartz sand at elevations, well above the regional groundwater table. There are steep hydraulic gradients between a number of the perched lakes and adjacent areas with lower elevations.
The perched lakes of Fraser Island preserve in their organic sediments, the record of changes to the island’s water table and the nature of the vegetation stabilizing the ancient dunes, through glacial and interglacial cycles of the Quaternary period to the present day.
Some of the lakes might result from slow drainage of rainwater through weathered sands.
The long term succession of the forests on the aging dune soils, periodically halted by the cold dry glacials, is illustrated by the pollen assemblages preserved in accumulated sedimentary layers of the lake basins.
Some of the lakes are believed to be several hundred thousand years old and contain some of the oldest known continuous lake sedimentary records in Australia and probably the oldest known coastal record of climatic change during the ice age in the world.
The lakes are highly oligotrophic, containing very small quantities of nutrients due to the infertile sands around them. They are well oxygenated with very dilute (average salinity 40 mg/L) acidic waters (pH=3.0 — 6.5) containing high proportions of allochthonous organic material.
The lake sediments consist of organic mud. Research work at Hidden Lake and a nearby ephemeral lake basin suggests the deposits date back to at least about 26,5080 (plus or minus 1260) [ANU – 5505] and about 300,000 years before the present respectively. The ephemeral lake has 6 metres of deep water organic lake sediments.
The brown colouration of perched lakes (due to suspended of fine organic colloids) severely limits penetration of sunlight, restricting photosynthetic growth to the surface waters. Limited photosynthesis, coupled with low concentrations of nutrients, results in low primary and secondary productivity, and perhaps also in low biotic diversity. However, the fauna has distinctive characteristics and, at least 12 invertebrate species which are unique to these dune lakes.
The mode of origin of the perched lakes, their distinct physico-chemical features and unique faunal elements sets them apart from other lake types in Australia and from dune lakes of the northern hemisphere.