Geology
The basin began to form in the early Paleogene (about 60 million years ago) when south-eastern South Australia started to sink and rivers began to deposit sediment into the large, shallow basin. The basin is still gradually sinking, and still gradually accumulating sediment. For many millions of years, the Lake Eyre Basin was well supplied with water and largely forested. About 20 million years ago, large shallow lakes formed, covering much of the area for about 10 million years. From that time on, as Australia drifted further north and the climate became gradually more arid, the lakes and floodplains started to dry. Only in the last 2.6 million years did the onset of the ice ages bring about the present climatic regime and the consequent fairly rapid desertification of the area.
Significant minerals deposits such as oil and natural gas, including Australia's most significant onshore petroleum reserves, are found within the basin. The mining and petroleum industries account for the greatest economic activity in the Lake Eyre Basin. Opals, coal, phosphate, gypsum and uranium are also mined from the basin. In 2009, the Queensland Environmental Protection Agency confirmed that heavy metals from mining operations near Mount Isa had entered the upper reaches of the Georgina River. The spill has the potential to contaminate parts of the basin as far south as Lake Eyre.
Geography
The Grey Range is a low-lying range of hills located primarily in Queensland. It stretches from the west of Blackall in Central Western Queensland to the north of Tibooburra in the far west of New South Wales. The range's highest point, Mount Arrowsmith, reaches 2,000 ft (610 m) above sea level. Comprising a series of low peaks, Grey Range has an average elevation of 1,150 feet (350 metres).
During years of especially high rainfall, all the riverbeds in this vast, mostly flat, arid and semi-arid area lead inland (not towards the sea) towards Lake Eyre in central South Australia. A river system is composed of the river and its tributaries. The watershed which feeds the river is also considered part of the river system. Areas of flooding, such as flood plains and the delta, can also be parts of the river system. A divide is an area of high elevation which separates watersheds of rivers. The divide is generally a large mountain range which does not allow water to flow from one watershed to another.
Lake Eyre itself lies approximately 15 metres below sea level, and usually contains only salt. In flood years it fills and for a short time undergoes a period of rapid growth and fertility: long-dormant marine creatures multiply and large flocks of waterfowl arrive to feed and raise their young before the waters evaporate once more. The annual mean runoff in the Lake Eyre Basin is lowest of any of the world's major drainage basins.
None of the creeks and rivers in the Lake Eyre Basin are permanent: they flow only after heavy rain – a rare to very rare event in the arid interior of Australia. These waterways by definition are therefore ephemeral waterways. An ephemeral stream is a temporary stream that only flows for a brief period as a direct result of precipitation. It occurs mainly in arid and semi-arid regions where rainfall occurs infrequently. The word, ephemeral, is derived from the 16th Century Greek word "ephēmeros," which aptly means "lasting for a very short time."
Average annual rainfall in the area surrounding Lake Eyre is 125 millimetres (5 inches), and the pan evaporation rate 3.5 metres (about 11 feet). Annualised average figures are misleading: since 1885 annual rainfall over the 1,100,000 square kilometres of the Lake Eyre Basin has ranged from about 45 millimetres (less than 2 inches) in 1928 to over 760 millimetres (30 inches) in 1974. Most of the water reaching Lake Eyre comes from the river systems of semi-arid inland Queensland, roughly 1000 kilometres to the north.
To provide a sense of scale, the Lake Eyre Basin is about the size of France, Germany and Italy combined. It is roughly the same size as the Murray-Darling basin (which drains inland eastern Australia and is responsible for a large proportion of the continent's agricultural productivity) but has vastly less water. Nevertheless, the entire flow of the Murray-Darling would be insufficient to fill Lake Eyre, merely keeping pace with evaporation.
Logging Tasks
Please email me via the GC.com message centre or here, your responses to the following questions.
1. Describe what you understand an ephemeral river or lake system to be.
2. Do you see any evidence of rainfall here, describe that evidence.
3. At the posted coordinates, what is the elevation here, and using the information provided in the body of the text, is the elevation here above or below the average height for the Grey Range?
4. Include a photo of yourself or team name with the tower in the background. (Please be careful to not include spoilers in this photo).