Lake Alexandrina (South Australia)
Lake Alexandrina is the largest of the Lower Lakes
|Primary inflows||Murray, Bremer, Angas, and Finniss Rivers|
|Primary outflows||Murray Mouth|
|Catchment area||1,061,469 square kilometres (409,835 sq mi)|
|Surface area||64,900 hectares (251 sq mi)|
|Average depth||2.8 metres (9.2 ft)|
|Max. depth||6 metres (20 ft)|
|Water volume||1,610 gigalitres (0.39 cu mi)|
|Surface elevation||0.75 metres (2.5 ft)|
The lake was named after Princess Alexandrina, niece and successor of King William IV of Great Britain and Ireland. When the princess ascended the throne and took the name Queen Victoria there was some talk of changing the name of the lake to Lake Victoria, but the idea was dropped.
Lake Alexandrina is located north of Encounter Bay and east of Fleurieu Peninsula. The Murray River is the major river to flow into Lake Alexandrina. Others include the Bremer, Angas, and Finniss Rivers, all from the eastern side of the southern Mount Lofty Ranges. The lake is shallow and contains a number of islands near the southern end.
The lake empties into the sea near Goolwa (the channel is known as the Murray Mouth), but when the river flow is low the entrance is often blocked by a sand-bar. Originally subjected to tidal and storm inflows of seawater the lake is now maintained as fresh water by a series of barrages across the islands near the Murray Mouth.
Though connected to the ocean historically the fresh and salt water flows mixed very little, with the lake area remaining fresh over 95% of the time with normal river inflow. Salt water inflows from the ocean would result in relatively little mixing of fresh and salt water, either vertically in the water column or laterally across the flow stream. Hindmarsh Island is reputed to be the largest island in the world with salt water on one side and fresh water on the other. Lake Alexandrina is connected by a narrow channel to the smaller Lake Albert to the south-east.
Edward Wilson, visiting the lake in the 1850s described it as follows:"Lake Alexandrina is the finest sheet of fresh water I ever saw. Indeed so formidable did it look, with a stiff wind blowing up quite a sufficient swell to make one seasick, that I could scarcely believe it to be fresh. Such is the fact however. It is forty or fifty miles long by twelve or fifteen wide and the shores around it receded into the dim distance until they become invisible, in the way which we are accustomed only with ideas of salt water. Supplied almost entirely by the Murray, the whole lake retains the muddy tinge of which I have spoken, and this sadly detracts from the otherwise beautiful appearances of this magnificent sheet of water."
In 2008, water levels in Lake Alexandrina and Lake Albert became so low that large quantities of acid sulphate soils threatened to form. The soils on the lake beds are naturally rich in iron sulfides. When exposed to the air, such as may occur in a time of severe drought, the sulfides oxidize, producing sulfuric acid. The barrages now prevent seawater inflows that have prevented this phenomenon in every drought since the last ice age. A weir has been proposed near Pomanda Island to protect upriver water supplies should it become necessary to open the barrages.
Turtles live in the lake, with lizards and snakes present along the shoreline. Insect species include dragon flies, a range of moths and butterflies and large numbers of beetles (coleoptera). Freshwater fish inhabit the lake, including the introduced European carp. The soils around the lake are relatively low in organic carbon although good barley and vegetable crops may be produced. Non-wetting soils are present along the south eastern bounds of Lake Albert and in areas around Lake Alexandrina.
The lake is a habitat for many species of waterbird, including migratory waders, or shorebirds, which breed in northern Asia and Alaska. It forms part of the 1300 km2 Lakes Alexandrina and Albert Important Bird Area (IBA), identified as such by BirdLife International because it regularly supports critically endangered Orange-bellied Parrots, endangered Australasian Bitterns, vulnerable Fairy Terns, as well as over 1% of the world populations of Cape Barren Geese, Australian Shelducks, Great Cormorants and Sharp-tailed Sandpipers.
- "Alexandrina, Lake". Place Names of South Australia. State Library of South Australia. Retrieved 2007-01-12.
- Wilson, Edward (1859). Rambles at the antipodes: a series of sketches of Moreton Bay, New Zealand, the Murray River and South Australia and the overland route. With 2 maps; and 12 tinted lithographs, illustrative of Australian life. W.H. Smith. p. 45. Retrieved 2 January 2011.
- "Catalyst - Fire, Flood and Acid Mud". Retrieved 2008-05-07.
- "IBA: Lakes Alexandrina and Albert". Birdata. Birds Australia. Retrieved 2011-08-03.