The Western Plateau
Sometimes called the Australian Shield, the
Western Plateau occupies more than half of the continent, including all
of Western Australia, nearly all of the Northern Territory, and more than
half of South Australia. It contains most of Australia's deserts and salt
lakes, weird rocks and grotesque hills, and many of its mines. It is sparsely
inhabited. Topographic monotony, the result of millennia of weathering
and erosion, is its most outstanding characteristic. Most of the plateau
is 1,000 to 3,000 feet (300-900 meters) above sea level, and many of the
higher points are isolated residuals, remains of landmasses that have been
eroded; Mount Zeil, at 4,957 feet (1,511 meters), in the MacDonnell Ranges,
is the highest point. Coastal plains are discontinuous and generally narrow.
At least half of this vast region receives less than 10 inches (250 mm)
of rainfall annually, and only in the northern and southwestern margins
does rainfall exceed 25 inches (635 mm). Because of this paucity of rainfall
and the general lack of relief, there are very few rivers in the interior,
and those that do exist do not reach the sea. The many lakes shown on maps
are normally dry salt or clay pans, the foci for the interior drainage
basins. Even around the continental margins most rivers are intermittent
and vary greatly in their seasonal flow.
Most of the interior is an even to broadly undulating surface broken by occasional rocky ranges and remnants of landmasses. Four particularly desolate areas are accorded individual names: the Great Sandy Desert, the Tanami Desert, the Gibson Desert, and the Great Victoria Desert. Here thousands of parallel ridges of red sand 30 to 50 feet (9 to 15 meters) high stretch unbroken for as much as 100 miles (160 km). The most significant relief features of the interior section are the MacDonnell Ranges in the Alice Springs district and the Musgrave Ranges on the border between the Northern Territory and South Australia. The most famous high points, located just north of the Musgrave Ranges, are Mount Olga, Ayers Rock, and Mount Conner. Vegetation over most of this region is sparse, consisting largely of grasses, acacia trees, and desert shrubs; after rains a profusion of herbaceous plants may spring up briefly.
The southern margin of the plateau forms the Nullarbor Plain, composed of thick beds of up to 800 feet (245 meters) of almost horizontal marine limestones. Beginning near Point Fowler in South Australia and running west for more than 600 miles (965 km), steep, often sheer, limestone cliffs rise as much as 200 feet (60 meters) from the ocean to the edge of the plain. The plain then, for about 150 miles (240 km) inland, rises gently northward to elevations slightly below 1,000 feet (300 meters). The flatness of the Nullarbor is indicated by a stretch of the transcontinental railroad that has no bend for 300 miles (480 km). The limestone permits easy entry of the annual 8 inches (200 mm) of rain, and there are no lakes or surface drainage. However, subsurface drainage has fashioned a bewildering maze of caverns and underground channels that honeycomb the limestone. The lack of water and the sparseness of the vegetation make the Nullarbor Plain one of the emptiest parts of the continent. The Barkly Tableland in the Northern Territory, of about 50,000 square miles (129,500 sq km), is another major plain at least partially underlain by limestone. This broad, open, and gently undulating plain has an average elevation of 850 feet (260 meters). The average annual rainfall is about 15 inches (380 mm), sufficient to maintain open grassland, on which there is extensive cattle ranching.
The most rugged portion of the shield area is in the Kimberley District of Western Australia, where strongly folded and elevated ranges have combined with an annual rainfall of more than 30 inches (750 mm) to produce substantial relief. The Arnhem Land region of the Northern Territory, an uplifted block cut by unusually long and straight fractures, is also much broken, although most of it lies below 1,000 feet (300 meters). In both regions the vegetation is eucalypt woodland with considerable areas of tropical savanna grassland. Both are among the most remote and inaccessible parts of the continent.
The Western Plateau has two areas of special economic importance. The southwestern corner is the only portion of the shield where climate and soils permit temperate land agriculture. This small region provides land both for grazing and for the cultivation of almost all of Western Australia's wheat, orchard and vine crops, and vegetables. It also supports Perth, the only large city of the entire plateau. The Pilbara Districts, inland from Dampier and Port Hedland, has immense reserves of high-grade iron ore. It is an elevated but much dissected portion of the plateau with an average elevation of about 2,500 feet (750 meters).
The Eastern Highlands
Along the eastern coast of Australia from
Cape York through central Victoria and including Tasmania is a belt of
highlands 50 to 275 miles (80-445 km) wide. They contain about 500,000
square miles (1,295,000 sq km) of the most varied terrain on the continent.
The traditional name for this elevated zone, the Great Dividing Range,
is inappropriate, for in no sense is there a continuous range, rarely are
there true rangelike characteristics, and nowhere are elevations really
great. Although the major east-west continental divide lies within it,
in many places the actual divide is indistinguishable. Except on Cape York
Peninsula, the rock materials are sediments deposited in the Tasman Geosyncline
between the early Paleozoic and the Cretaceous periods, overlain by large
amounts of volcanic materials.
Elevations within the Eastern Highlands vary considerably. They are lowest on the discontinuous coastal plain that borders the east and southeast coast; except at river mouths these plains are rarely more than 10 miles (16 km) in width. Low hills frequently break the surfaces of the plains, and a rather hilly zone several miles wide often intervenes between the plain and the steep, seaward-facing slopes that mark the beginning of the highlands proper. These slopes are much steeper than those facing inland, and in some areas they are so close to the coast that spurs form cliffed headlands on the Pacific Ocean. In the north the highest points are on the eastern margin of the Atherton Tableland, where Bartle Frere reaches 5,322 feet (1,622 meters). But from there south to Brisbane little of the highlands rises above 2,000 feet (600 meters) above sea level, and much of it lies below 1,000 feet (300 meters). Elevations again increase to above 5,000 feet (1,500 meters) in the New England Range and remain above 2,500 feet (750 meters) through the Blue Mountains to culminate in the Snowy Mountains, which contain the highest elevation -- 7,310 feet (2,228 meters) -- on the continent.
The Eastern Highlands have two different types of drainage. Most streams flowing to the coast are perennial. Many rise west of the highest part of the highlands, and drainage patterns are complex. Some streams have cut deep gorges, and there is great potential for water storage and power. South of Toowoomba the opposing west-flowing streams form part of the continent's largest integrated drainage system, the Murray-Darling. They rise less than 100 miles (160 km) from the east coast, and many are permanent only at their sources.
In the Cape York Peninsula, the northernmost portion of the Eastern Highlands, the divide lies within 15 to 20 miles (24-32 km) of the east coast and rises to between 1,600 and 2,000 feet (490-600 meters). Vegetation is principally closed eucalypt woodland with patches of heavy rain forest. The area is remote and difficult of access.
The northernmost plateau-like portion of the highlands is the Atherton Tableland, an elevated area of about 12,000 square miles (31,000 sq km) inland from Cairns. The rise from a tropical coastal plain to the plateau surface between 3,000 and 4,000 feet (900-1,200 meters) above sea level is abrupt, and this, in a region of moist onshore winds, is responsible for the district's comparatively heavy rainfall. The uneven surface has fertile volcanic soils on which dense rain forest once grew. The region still has commercially important hardwoods. However, most of the forest has been cleared, and the tableland is now cultivated.
South of the Atherton Tableland the divide swings inland but maintains an average elevation of about 2,000 feet (600 meters) to the vicinity of Hughenden, where it loses all semblance of a highland. For more than 500 miles (800 km) the divide lies more than 250 miles (400 km) from the east coast, its most inland position. The intervening Bowen Basin yields large quantities of coking-quality coal. West of Toowoomba are the Darling Downs, an area of rolling country, which lie entirely west of the divide. Their volcanic soils constitute the best temperate climate agricultural land in Queensland.
For 325 miles (525 km) between Toowoomba and the Hunter River valley, the Eastern Highlands broaden and rise to form the New England Tableland, the largest and most discrete of the elevated plateau-like areas. It contains about 16,000 square miles (41,400 sq km) of smoothly contoured but hilly terrain, rising in places to as much as 5,100 feet (1,600 meters) above sea level. Throughout the tableland the divide lies between 45 and 80 miles (70-130 km) from the east coast. The highest points are less than 20 miles (32 km) from the sea. Descent to the narrow and often hilly coastal plain is abrupt, and the slopes are covered with temperate rain forest. Much of the original eucalypt woodland and grassland has been cleared for pasture.
The Blue Mountains is an area of steep east-facing slopes, located inland from the Cumberland Plain west of Sydney. Spectacular gorges and waterfalls have resulted from erosion by such coastal rivers as the Shoalhaven and Hawkesbury, and the district, still largely covered with heavy eucalypt forest, is a major recreation area. The main section of the highlands proper lies about 100 miles (160 km) from the coast and centers around the city of Bathurst, which occupies a broad basin surrounded by hills at elevations of 4,000 to 4,500 feet (1,200-1,350 meters). To the south a lower portion of the highlands is centered on Goulburn. Canberra lies on the southern margin of this hilly but open plateau, most of which is devoted to sheep raising.
The highest portion of the Eastern Highlands forms an arc about 180 miles (290 km) long south and southwest of Canberra. Although the area is called the Australian Alps, even the highest peaks, which are above 6,000 feet (1,850 meters), are merely remnants of former landmasses, rising above a series of severely dissected plateaus. However, the terrain is locally very rugged. The Snowy Mountains is the only part of the mainland where substantial snowfalls occur yearly; it is the site of the Snowy Mountains Hydro-Electric Scheme, which provides water for power and irrigation in the Murray and Murrumbidgee valleys. On the inland side the lower slopes have been largely cleared for sheep raising; the higher and seaward slopes are covered with closed eucalypt forest to elevations of about 6,000 feet (1,850 meters), above which are alpine meadows. South of the main highlands in Victoria is the Gippsland district, a much dissected foothill zone once covered with heavy temperate rain forest. Much of this is now cropland and pasture, but an active lumber industry remains. In Victoria the Eastern Highlands trend east-west and are known locally as the Victorian Highlands. They continue westward almost to the South Australia border, maintaining elevations of above 3,000 feet (900 meters) nearly to the end. The area is a prosperous grazing and wheat-growing district.
Tasmania, as well as the major islands in Bass Strait, is a portion of the Eastern Highlands that separated from the mainland. It is a somewhat undulating plateau with an average elevation of 3,000 to 4,000 feet (900 to 1,200 meters), above which a few eminences rise another 500 to 1,300 feet (150-395 meters). A number of large shallow lakes and many smaller ones are found on the plateau, and some of the lakes have been developed for hydro-electric power. The central plateau is surrounded by rugged terrain much dissected by streams that flow from the interior; some parts of the southwest are almost unexplored. Heavy temperate rain forest grows in the west and south, but has been cleared along the north coast and in the lowland corridor, the Midlands, between Launceston and Hobart. Fruits, especially apples, and sheep are raised on the island.
The Central Lowlands
About one third of Australia lies in the
Central Lowlands, a broad open corridor between the Eastern Highlands and
the Western Plateau. Structurally it is a series of basins with sediments
overlying crystalline rocks. Remnants of old mountains around which later
sediments were deposited, such as the Mount Lofty, Flinders, and Main Barrier
ranges, are along the lowlands margin and, in places, within the lowlands
itself. Lack of relief and rainfall are the most outstanding characteristics
of the lowlands. Very little of it rises higher than 1,000 feet (300 meters)
above sea level, and most areas are below 500 feet (150 meters). The highest
areas occur where the lowlands border the Flinders Ranges and the Eastern
Highlands. About 4,000 square miles (10,400 sq km) around and including
Lake Eyre lie below sea level. Most of the lowland terrain is gently and
monotonously undulating; the only relief features are flat-topped but steep-sided
erosional remnants rising a few hundred feet above the plain. The vast
bulk of the lowlands receives no more than 15 inches (380 mm) of rain,
and includes the continent's driest area, around Lake Eyre, where annual
averages are below 5 inches (125 mm). Low divides separate the lowlands
into three main basins. In Central Queensland an indistinct divide joins
the Eastern Highlands to the Western Plateau, separating the (Carpentaria)
Gulf Plain from the Lake Eyre Basin. Farther south an equally low divide
sets off the Murray-Darling Basin.
Even and almost featureless, the Gulf Plain is sharply limited in the west by the rough, rocky, highly mineralized country in the Cloncurry-Mount Isa district and on the southeast by the Eastern Highlands; some 300 miles (480 km) south of the coast a low divide forms the southern boundary. Streams with low gradients, such as the Gilbert, Flinders, and Leichhardt, flow into the gulf, and large areas are subject to flooding. Soils support open eucalypt woodland and grass. Rainfall in the Gulf Plain is the highest and most reliable of any portion of the Central Lowlands, ranging from about 15 inches (380 mm) annually on the divide to 39 inches (970 mm) on the gulf. The Gulf Plain is mostly cattle pasture.
South of the Gulf Plain and the divide the lowlands broaden greatly to a maximum of 750 miles (1,200 km) across southern Queensland and northeastern South Australia; the maximum north-south extent is about 700 miles (1,130 km). This large area is entirely one of interior drainage, and it comprises several drainage basins, the largest of which is that of Lake Eyre, with 441,600 square miles (1,143,700 sq km). The Lake Eyre basin, which includes a large portion of the Simpson Desert, receives drainage from many markedly intermittent streams. Gradients are so low that in some cases large rivers spread out over the countryside, eventually to reform, sometimes under a different name. In this way the Thomson and Barcoo, which rise in the Eastern Highlands, become Cooper's Creek; the Diamantina with its major tributaries, the Hamilton and Georgina, becomes the Warburton. At rare intervals drainage from the plateau to the west may reach Lake Eyre via the Macumba and the Neales. These streams are normally a maze of dry channels bordered by eucalyptus. Occasional deep stretches form valuable permanent waterholes. Water does not flow in these channels every year. When it does, it originates as heavy, sometimes tropical storms on the higher land to the east and north. The resulting floods spread out widely and may take weeks to move down stream. Such flooding brings bountiful temporary pasturage, but the floods cannot be depended upon. The part of the lowlands where South Australia and Queensland meet is utilized for grazing; the area around Lake Eyre is virtually unoccupied. The Great Artesian Basin underlies most of this region and provides the water that makes grazing possible.
In the southeast section of the Central Lowlands is the Murray-Darling Basin, containing the continent's largest integrated drainage system. Here also is a vast tract of lowrelief terrain drained by streams and rivers with very irregular flow. In spite of the large areas drained -- 414,200 square miles (1,072,800 sq km) -- and the length of the main rivers, the discharge of the system is not large. Rising in the Eastern Highlands, the Murray and Darling rivers flow west and southwest through low areas with little rainfall and high rates of evaporation. These factors, combined with extensive meandering, cause the rivers to lose volume throughout most of their courses.
The area drained by the Darling River is basically sheep pasturage, with mixed grain farming and sheep raising in the eastern portions. The Riverina district, lying between the Lachlan and Murray rivers, together with lands along the lower Murray and Victorian tributaries, constitute Australia's most important grazing and agricultural districts. Terrain and soils are suitable for large-scale irrigation. The most important districts are the Murrumbidgee Irrigation Districts, between the Murrumbidgee and Lachlan rivers; the Riverina Irrigation Districts, on the New South Wales side of the Murray; and the Goulburn-Campaspe-Loddon districts in Victoria. There are also several smaller irrigation areas on the lower Murray. Livestock is raised in these areas, and orchard and vine crops and a variety of vegetables are grown. Implementation of the Snowy Mountain Hydro-Electric Scheme has brought additional water into the Mur ray and Murrumbidgee system; still, there is far more land suitable for irrigation than there is water.
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The principal river system of Australia is
the Murray, which together with its major tributaries -- the Darling,
Murrumbidgee, and Goulburn -- drains an area of 414,200 square miles
(1,072,800 sq km) in New South Wales, Victoria, Queensland, and South Australia.
Headwater streams of the main tributaries lie within 125 miles (200 km)
of the east coast and join to form major rivers that flow by roundabout,
often meandering, channels to the sea at Encounter Bay, South Australia.
The Murray River rises in the Snowy Mountains and has a length of 1,600
miles (2,575 km), of which the lower 600 miles (970 km) are navigable by
small vessels; sandbars across its mouth prohibit the entry of coastal
vessels. The Murrumbidgee, which has a length of 1,050 miles (1,690 km),
rises near Cooma and joins the Murray near Boundary Bend. The constancy
of flow in both the Murray and Murrumbidgee rivers is assured by the Snowy
Mountains Hydro-Electric Scheme. The Darling's tributaries drain the entire
western slope of the Eastern Highlands in northern New South Wales and
part of southeastern Queensland. The main stream of the Darling is 1,700
miles (2,740 km) long and joins the Murray at Wentworth. Dams on the main
stream and several major tributaries maintain flow except during the most
Over slightly more than half of the continent drainage is uncoordinated or goes to interior drainage basins. On the Western Plateau drainage is uncoordinated, and such streams as exist flow briefly at rare intervals and end in temporary lakes or swamp basins with no outlet. Another 441,600 square miles (1,143,700 sq km) of Queensland, the Northern Territory, and South Australia drain to Lake Eyre, one of the world's largest single basins of interior drainage. The major rivers of the Lake Eyre Basin -- the Georgina, Diamantina, and Cooper's Creek -- have very low gradients and are normally a maze of dry, intertwining channels; yet after rains they may be many miles wide. The water very rarely reaches Lake Eyre: In 1950 the basin filled for the first time since European settlement.
Because the flow of Australian rivers is highly variable, they are difficult to harness. Dam sites are few, especially in the interior, and large storages must be provided to ensure constant supply. Evaporation losses are also high, especially in the driest regions. Only in Tasmania is stream flow reasonably dependable in all seasons.
Most of Australia's lakes are waterless expanses of saline clay. On the rare occasions when they do contain water, it is muddy, salty, and shallow. The Western Plateau of Western Australia contains many such lakes, but the largest are in South Australia -- lakes Eyre, Torrens, Gairdner, and Frome. Along the southeastern coast of Australia are numerous brackish or saltwater lagoons cut off from the sea by sandbars. The largest freshwater lakes are in Tasmania, where several, including Great Lake, are used to generate hydroelectricity.
The supply of underground water is of vital
importance to much of pastoral Australia. More than 1,250,000 square miles
(3,240,000 sq km) of the continent is underlain by basins that yield water.
Most of this groundwater contains dissolved solids that are harmful to
crops, but in most cases the water is usable for livestock.
The Great Artesian Basin, the world's largest artesian basin, underlies 676,250 square miles (1,751,480 sq km) in Queensland, South Australia, New South Wales, and the Northern Territory. Although the water is often hot and has a high mineral content, the region's pastoral industry depends on it. Smaller artesian basins are found in Western Australia and southeastern Victoria.
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As a compact landmass Australia creates some of its own wind patterns, but they yield little moisture. The bulk of the continent is located in the southern hemisphere subtropical belt of high pressure that centers at about 30° south latitude, and during much of the year dry winds blow out of the heart of the continent; this condition is most persistent in winter (May-September). In summer low pressure develops over the Kimberleys in the northwest, attracting warm moist winds, called monsoons, from the Timor and Arafura seas and thus bringing rain to northern Australia. Southerly winds blow parallel to the west coast most of the year, leaving it one of the world's driest coasts. In winter cyclonic storms affect the southern fringes of the continent and Tasmania. The east coast north of Newcastle is in the path of the southeast trade winds, which bring moist air to the coast; as this air rises over the margin of the Eastern Highlands it often produces substantial rainfall. Occasional tropical cyclones (hurricanes) sweep in from the northeast, bringing much destruction to the east coast between Cooktown and Brisbane. These intense traveling low pressure systems also strike the northwest coast between Derby and Port Hedland, where they are known as "willy-willys."
Australia's reputation as the dry continent
is well deserved. Nearly 40 percent of the continent receives less than
10 inches (250 mm) of precipitation annually, and about 70 percent receives
less than 20 inches (500 mm); the latter figure generally marks the limit
below which most crops cannot be grown without irrigation. The driest area
centers on Lake Eyre in South Australia, where a few thousand square miles
receive less than 5 inches (125 mm) annually. Over a much larger area in
central Australia useful rain may not fall for several consecutive years.
Areas of high precipitation are small and are associated with marked lifting of moist air over topographic barriers. The highest total on the continent occurs in a small area near Tully, Queensland, where lifting of moist air over the eastern margin of the Atherton Tableland brings a total of 177 inches (4,500 mm) annually. Only the far northern coast, the east and southeast coast, the far southwestern corner, and Tasmania consistently have annual rainfalls of more than 20 inches (500 mm). Snow regularly falls only in two areas: above 4,500 feet (1,350 meters) in the Australian Alps of Victoria and New South Wales; and above 3,500 feet (1,050 meters) in the highlands of Tasmania. In some years snow may fall on the New England Tableland. The snowfall of the Australian Alps is of considerable economic importance, because it stores water that ultimately flows to the Snowy Mountains Hydro-Electric Scheme.
Over much of Australia there is a marked seasonal variation in rainfall. On all of the continent north of the Tropic of Capricorn, as well as the entire east coast south to the Victorian border, most of the rain comes in summer (December-March). In the far north more than 85 percent of the annual precipitation may fall during the first three months of the year. In southern Australia and on the west coast north to Exmouth Gulf the rainfall is sharply restricted to the winter months; at Perth 80 percent of the rain falls between the beginning of May and the end of September. The drier months may be virtually rainless, and grazing animals suffer greatly from lack of feed and water.
Much of Australia also experiences high variability of rainfall; that is, in any given year there is likely to be a substantial deviation above or below the average. Variations above the average may bring local flooding, but variations below the norm may be disastrous, especially in areas where the annual total is small. Even more serious is the occurrence of several consecutive years of below average rainfall. Widespread drought is common in the Australian Interior, bringing death to livestock, financial ruin to graziers, and destruction even of the xerophytic vegetation.
Australia is generally thought of as a hot
continent, but it is, in fact, somewhat cooler than most other continental
areas at comparable latitudes of the southern hemisphere. Seasonal variations,
on the whole, are small. Generally, coastal and highland areas, especially
in the southeast, are cooler than interior locations, and the north, particularly
the northwestern coast, is the hottest region.
During the summer season, between December and March, almost any place on the mainland may experience temperatures above 90°F. (32°C.), and readings above 100° F. (38°C.) are common. In the interior temperatures may rise above 115°F. (41°C.), and 60 consecutive days with temperatures above 100°F. is not uncommon. In the coastal areas of the south and east, heat waves resulting from strong winds from the interior last several days. Darwin has an average January mean of 84°F. (29°C.), while Melbourne has one of 68°F. (20°C.) and Sydney one of 72°F. (22°C.). Alice Springs, in the interior, has a January mean of 83°F. (28°C.), and Perth, in the southwest, one of 74°F. (23°C.).
Although extremely low temperatures are uncommon, there are few areas where frost may not occur during the winter, and in the southeast frost limits the cultivation of many crops and pasture plants. The major frost-free areas are in the Northern Territory and Queensland north of the Tropic of Capricorn, and along the entire coast north from Shark Bay in Western Australia to Brisbane on the east coast. About two thirds of the continent has an average of 300 or more frost-free days. The Eastern Highlands in New South Wales and Victoria, the Australian Alps, and most of Tasmania may have frost at any time of the year. Average July mean temperatures in the southeast are 49°F. (9°C.) in Melbourne and 53°F. (12°C.) in Sydney. In the north the July mean is 53°F. at Darwin. In the interior Alice Springs has a July mean temperature of 77°F. (25°C.).
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