Vergemont Creek flood. Photo: A Emmott
Water is always critical to what happens in desert environments, and nowhere is this demonstrated more vividly than in the Lake Eyre Basin. Other parts of the Australian desert get occasional inundations of rainfall, however, nowhere else does the water from several rivers that rise hundreds of kilometres away, channel into an internally draining (endorheic) system. The fate and role of water in the system is determined by the amount of rain that falls and the topography of the landscape. In this wide, shallow basin, with very little slope, many rain events only yield enough water to provide sustenance for local plants and animals, top up isolated waterholes and billabongs, and generate minor flows in the major river channels.
However, with the intense heavy rainfalls that can come from northern monsoonal systems, the rivers rise rapidly, connecting isolated waterholes along main river beds, and then spreading out across the landscape into a completely different network of channels, extensive braided streams, floodplains and swamps. These huge flows can re-direct channels and carve out new ones. The floodwaters move toward Kati Thanda - Lake Eyre, but due to the low gradient, can take anywhere from 3 to 10 months to reach the vast salt lake, if at all. Most of the water is lost by evaporation and transpiration.
Goyder Lagoon, an extensive wetland area of around 1300 km2, is fed directly by both the Diamantina and Georgina Rivers. The complex networks of braided channels receive water more regularly than other parts of the Basin and host millions of breeding water birds. The surrounding gibber plains support several small mammal species, including the endangered Kowari,
Catchments in the Basin
The regional patterns of water distribution across this huge area are influenced by the landforms and slope, which determine the natural direction of surface water flow. These topographic features naturally divide the Basin into distinct catchments.
These are the
Cooper in the north-east, the
Georgina and Diamantina (in the north), the
Todd, the Hay and the Finke (in the west), and
Lake Frome (in the south and south-west).
Rain falls most often in those catchments with headwaters in the north of the Basin. Although rainfall is often higher in the Cooper than in the Georgina and Diamantina, the crystalline rocks and lower vegetation cover of these two catchments give rise to more runoff. On average, the Diamantina is the most regular contributor to Kati Thanda - Lake Eyre, about every 2 years, followed by the rivers to the west of Kati Thanda - Lake Eyre (Macumba, Neales and Peake) every 4 years, and the Cooper every 6 years. The Lake only fills about once every 10 years. In modern times, the Hay, Todd and Finke catchments do not contribute to Kati Thanda - Lake Eyre, but probably did so in the past. Creeks and waterways draining to Lake Frome do not contribute to Kati Thanda - Lake Eyre.
Across the catchments, some areas are characterised by deep, steep-sided waterholes, billabongs and creeks, waterways and rivers meander across the country or feed into extensive lake systems. In other areas, the rivers just disappear, soaking into the desert sand. It is this range of water body types, along with low or high water flows occuring at any time of the year, that generate the ever-changing mosaic of aquatic ecosystems that gives the Lake Eyre Basin its extraordinary ecological importance. The maintenance of this mosaic also depends critically on the connections that come and go within the system, between upstream and downstream, or across the landscape between rivers, waterholes and swamps.
Shallow saline groundwaters are a feature of the Lake Eyre Basin, and salinity levels of surface waters are determined by high evaporation rates, seepage from groundwater and release of salt from soil and floodplain sediments.
Salinity of water bodies varies depending on location and season. Where rain occurs more often, in the upper reaches of the Cooper, Georgina and Diamantina catchments, many waters are fresh, but in the flat lands of South Australia, salinity levels are much higher. Due to the fine sediments, the rivers and waterholes are often unusually turbid, giving the water its typical brown colour. Nutrient levels are not unusually high, but are occasionally enhanced by roosting waterbirds.
Underneath most of the Lake Eyre Basin, and extending beyond it, lies Australia's most important groundwater resource, the
Great Artesian Basin. Grazing activity, town water supplies, mining and petroleum operations all depend on water extracted from this Basin using artesian bores.
This underground water also supplies unusual ecological systems in the Lake Eyre Basin called mound springs, estimated to be up to 700,000 years old. These systems support a specialised suite of plants and animals and the conical shape of many adds a distinctive aspect to the landscape. The water temperature varies from 20-45Â°C and some are saltier than sea water. The best examples are concentrated in the north east and south west regions of the Basin.
Artesian water moves underground very slowly (1-5m/yr), predominantly from the north east to the south and south west region of the Basin, with water in the middle of the Basin nearly 2 million years old. Recharge, mostly along the eastern edge, is also slow, and overuse of this water in the past has reduced the flow of many bores and mound springs. In 1999, the
Great Artesian Basin Sustainability Initiative (GABSI) made resources available for a programme of bore rehabilitation and bore drain replacement i.e. piping and capping the bores. This programme continues and promotes careful management of this groundwater resource so critical to the future of the Great Artesian and Lake Eyre Basins.
Compared to those of coastal Australia or in other arid parts of the world, the rivers of the Lake Eyre Basin are relatively unspoiled. With the exception of a few minor local diversions for farms and towns, they flow freely, with no major storage areas.
There is very little artificial draining or filling of wetlands, and several wetlands are recognised for their high natural values, both at the national and international level. The aquatic systems are basically healthy, with little evidence of nutrient build up or contamination, and the relatively few exotic species are usually localised, although recent expansions of Redclaw Crayfish and Sleepy Cod in western Queensland catchments, as well as Gambusia in GAB springs, are cause for concern.
However, this low level of disturbance is increasing and development is encroaching. If not well managed, such changes can lead to the problems seen in the adjacent Murray-Darling Basin, where water diversion, land clearing and major water extraction have had serious consequences (see
Threats section). It is important that future management plans and practices incorporate the lessons learned in the Murray-Darling system. This will help protect the unusually dynamic patterns of water flow and distribution in the Lake Eyre Basin.
- So high is the unpredictability of wet and dry periods in this part of Australia that the variability of flow for rivers in the Lake Eyre Basin is higher than anywhere else in the world.
- Only 68% of the area of the Lake Eyre Basin delivers water to Kati Thanda - Lake Eyre.
- On average, only a tiny fraction of the rain that falls in the Basin will flow all the way to Kati Thanda - Lake Eyre.