Features and Origins of Natural Lakes

Australia is an ancient, weathered continent and the landscape has undergone considerable environmental changes since stretching and rifting from the supercontinent Gondwana in the Jurassic (150m.y.a.) and traveling northward with separation off of NZ 80-60 m.y.a, and finally separation of Antarctica 45 m.y.a. Large parts of the north-east and south-west of NZ are remnants of the former eastern margin of Gondwanaland, but the NZ landscape has also been strongly influenced by recent tectonic and volcanic activity, as well as glaciations during the Pleistocene.

The Australian climate has generally been warm and moist, even when joined to Antarctica, and the drying of the continent probably began about 35-24 m.y.a. Drying continued as Australia drifted northward and temperature gradients between the equator and the pole increased until about 2.5 m.y.a. when continent-wide climatic conditions became similar to the present day.

Lakes in both countries have varied form and origin, reflecting the predominant geological processes, hydrology, and climate. Australia has an arid interior with eco-regions spanning arid, temperate, alpine, subtropical, and tropical. Standing waters include freshwater lakes, saline lakes, coastal lagoons, and drinking supply reservoirs in dammed river basins. The New Zealand landscape is young and mountainous, leading to tremendous diversity of lake types with at least ten different possible mechanisms of formation. Most of the lakes have been formed comparatively recently, within the past 20 000 years.

Accompanying the aridification of Australia has been the formation of saline lakes. The largest lake in Australia is Lake Eyre, which fills intermittently following high rainfall in the humid subtropical catchments of the tributary rivers, including the Diamantina-Warburton Rivers and Cooper Creek. The Lake Eyre basin, covering 1.3 million km2, was formed via upwarping of the Central Lowlands near the South Australian Gulfs. Instead of draining to the sea, water was directed inland to the lake which has a current elevation of 15 m below sea level. The climate in the Basin is hot and dry, and evaporation exceeds rainfall in all months over the entire Basin.

Saline lakes are a common feature of the arid centre and southern coastal regions of Australia. The mechanisms by which salt entered the lakes

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Figure 1 Rainfall map of Australia (provided by the Australian Bureau of Meteorology and used with permission).

New Zealand Mean annual Rainfall (mm) 1950-1980

Annual rainfall (mm) _ " 4000.1 - 11000 2000.1 - 4000 1500.1 - 2000 1250.1 - 1500 1000.1 - 1250 750.1 - 1000 500.1 - 750 0 - 500 j ; No data

Figure 2 Rainfall map of New Zealand (courtesy of Wei Ye, International Global Change Institute, University of Waikato).

Annual rainfall (mm) _ " 4000.1 - 11000 2000.1 - 4000 1500.1 - 2000 1250.1 - 1500 1000.1 - 1250 750.1 - 1000 500.1 - 750 0 - 500 j ; No data

Figure 2 Rainfall map of New Zealand (courtesy of Wei Ye, International Global Change Institute, University of Waikato).

include (1) evaporation of relict sea water from incursions of the ocean during the Tertiary and Quaternary and subsequent precipitation of dissolved solutes; (2) dissolution from rocks of marine origin; (3) additions of ocean sprays, continental dusts, and atmospheric deposition; and (4) weathering of terrestrial rocks and minerals. In contrast to Australia, NZ has only one natural saline lake - Sutton Lake in arid Central Otago. The lake's elevated salinity is sustained by the windy maritime climate in which the rate of evaporation (700 mm per annum) exceeds rainfall (500 mm per annum).

Lakes of volcanic origin occur in the Atherton tablelands in north Queensland and in the south-east of Australia. The Southern Volcanic Hills in the southeast of South Australia and western Victoria are generally considered to be less than 10 000 years old. There is an older group of volcanoes to the north-west of Blue Lake that may have erupted between 1 000 000 to 20 000 years ago. The most famous of the south-east crater lakes is Blue Lake in Mount Gambier, so named because it is brilliant blue year-round. Blue Lake is fed by two major aquifers, has a mean depth of 70 m, and recent dating of sediments suggests that it was formed more than 28 000 years ago.

The Atherton Tablelands has several maar lakes, including Lake Barrine, Lake Eacham, and Lake Tinaroo, which are now included within the Wet Tropics World Heritage Area. Volcanic activity in the Atherton area occurred between 10 000 (Pleistocene)

and 2 million (Pliocene) years ago. During the explosion that formed Lake Barrine, the aboriginal stories describe the landscape as open scrub. The lake is now surrounded by tropical rainforest, and sediment pollen records confirm that the rainforest was formed on the Atherton Tableland only around 7600 years ago.

Fraser Island, an island off the Queensland coast, is the world's largest sand island of 124 km long and covering an area of 1630 km2. Fraser Island hosts three types of lakes: window lakes, perched lakes, and barrage lakes. Window lakes occur when the ground drops below the water table. Barrage lakes, as their name suggests, are formed by the damming action of sand blown by the wind, blocking the waters of a natural spring. The morphology of barrage lakes is dynamic, depending upon sand encroachment into these lakes. Perched lakes occur above the water table and are formed as organic matter and sand accumulate in shallow depressions to form an impervious base and eventually capture rain, creating a lake. Fraser Island has half of the world's perched dune lakes, including the world's largest perched dune lake -Lake Boomanjin, covering an area of 190 km2.

Volcanic lakes in NZ occur in three main areas of the North Island: the Taupo Volcanic Zone, the Auckland Volcanic District, and the Bay of Islands-Kaikohe District. Lake Taupo (with an area of 616 km2), the largest lake in New Zealand, was formed from a series of explosive volcanic eruptions, with the last Taupo Pumice eruption considered to have occurred around

186 ad Many lakes of the Rotorua region (Figure 3) were formed through collapsed calderas following massive eruptions, often acting in combination with lava flows blocking outflows.

Lake Rotorua, for example, was formed around 140 000 years ago from a collapsed caldera following eruption of Mamaku Ignimbrite. Nevertheless many other lakes in this region have been formed relatively recently, including Lake Rotomahana and smaller lakes of the Waimangu Valley, which filled following hydrothermal explosions associated with the Tarawera eruption of 1886. This eruption is evident as the volcanic tephra is found in the bottom sediments of Rotorua lakes such as Tarawera and Okareka (Figure 4). Glacial lakes are the most common of New Zealand's lakes, comprising the deepest lakes due to their formation in former glacial valleys, with outlets dammed mostly by moraine and alluvial outwash. Lake Hauroko in the south of South Island is the deepest New Zealand lake (462 m) but Lakes Manapouri and Te Anau also exceed 400 m depth.

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