FLOODS OF LAKE EYRE
Lake Eyre North
Lake Eyre North occupies an area of 8 430 km² and can contain 27.7 km3 of water at - 9.5 m ahd and an average depth of 3.3 m. The deepest region of Lake Eyre North is the eastern part of Belt Bay in which bottom levels just deeper than —15.2 m ahd were found - the lowest point on the Australian continent. The floor of the Lake is very flat with bottom slopes in the order of 2 x 10-5, so the definition of the exact location is rather difficult. The deepest points of Madigan Gulf and Jackboot Bay are —15.2 m and —15.0 m ahd respectively.
The easily recognisable features include the north-south Warburton Groove, 5 km wide and up to 0.6 m deep, and the 3 km wide and 0.2 m deep Kalaweerina Groove, originating at the mouth of Warburton and Kalaweerina creeks, and the Cooper Groove running from the mouth of the Cooper Creek to the north of Brooks Island. Recent satellite photographs reveal a considerable degree of complexity of the Lake Eyre bottom surface, with many probably short-lived alluvial features carved by major or local inflows.
Dulhunty (1982) distinguishes three major zones in Lake Eyre North: arid saline playa environment covering the northern two-thirds of the bed; arid terminal salina environment with hard salt crust in the southern part; and a 10-15 km wide east-west slush zone extending 80 km across the full width of the Lake.
Evidence from the western side of Lake Eyre suggests that the present salina depression is a structural feature formed by downfaulting in the earth's surface about 30 000 years ago which blocked the outlet to the sea. The western margin of the Lake bed is a steep escarpment which is still seismically active, and springs occur along north south lines in the Lake bed, presumably marking fault zones. About 5 m of sediment have been deposited in the Lake bed since this downfaulting, and as the climate became more arid, the finer sediments were blown off the Lake surface to form the dunes of the Simpson Desert (Twidale, 1975).
The shores of the Lake are well defined and consist of sand dunes, cliffs of eroded gypseous loam or low rocky escarpments. Dulhunty (1983) reports the occurrence of large sand mounds, 30-48 m high and 1-2.5 km wide along the northern shores, from Koorakarina Creek to Cooper Creek. Bye (1980) notes that the south-eastern coastline, which consists of sand cliffs, is being rapidly cut back, with an erosion rate in the order of 5 m per flooding.
Flanking the Lake on the east and north are generally north-south trending, parallel sand dunes of the Tirari and Simpson deserts. The country to the west is more diversified, and is composed of gibber flats, low hills and a few large sandy ephemeral streams. To the south, gibber plains give way to dune country as the Lake is approached.
A salt crust up to 460 mm thick (Dulhunty, 1977) covers much of Madigan Gulf and Jackboot and Belt bays during dry periods. Beneath the salt crust, groundwater and sediment combine to form muds which have the consistency of thick soup. The 2 500 km² of crust is virtually floating on this slush (Dulhunty R., 1975). Most of the other small playas have only a thin crust of salt and gypsum covering their surface. The 400 million tonnes of salt deposited in the Lake (Bonython, 1955), equivalent to 80 years of salt production in Australia, dissolves totally in time of major inflows (Dulhunty, 1974, 1977), as a result the filled Lake is deeper than the dry. The distribution of salt changes after major fillings, as can be seen by comparing 1972 Skylark photographs and 1983 Landsat photographs.
A salt balance could be useful in determining previous inflows to the Lake. The analyses taken between March and May 1977 indicated a mean Total Dissolved Solids value of 61 and 85 g/m³ for the Diamantina River at Birdsville and the Cooper Creek at Innamincka respectively. With the volumes of water involved in the big flooding it would represent an influx of some 3 million tonnes of salt, i.e. 0.75% of the total Lake salt content. Mabbutt (1977) estimates the period of accumulation represented by existing salt crust of Lake Eyre as 5 500 years, and details the possible causes of evident salt losses.
From the 1953 Diamantina flood it is known that the destination of waters coming from the Warburton Groove is initially the Belt Bay area which on filling may overflow into Jackboot Bay and then into Madigan Gulf (Bonython, 1963). The Warburton Groove is also a line along which the Diamantina waters mix with inflows from the western tributaries; these inflows can be clearly seen in different undertones in the water. The Cooper Creek discharges its waters along Cooper Groove to Madigan Gulf.
Torgersen (1984) notes that due to the low gradient of the bottom of Lake Eyre, the advance and retreat of Lake waters due to wind stress can expose or cover large areas of the Lake bed. Its surface slope can even exceed the bottom slope and thereby create a 'roving' lake, especially for water depths less than 0.5 m.
The presence of several old beach lines, 0. 7, 1.6 and 2.8 m above the 1974 water level, indicates the occurrence of previous unrecorded major inflows (Dulhunty, 1975). At this time the term 'full' should therefore be used cautiously in relation to Lake Eyre. The above levels would represent approximate storages of 35 km³, 48 km³ and 67 km³ respectively and the potential available storage to sea-level is bigger than 200 km³, i.e. almost seven times more than the 1974 storage. In human dimensions the Lake was full several times in the past century, but taking into account its present geological shape it was possibly never totally filled in the past millennia.
'Filling' of the Lake by inflows of the order of 5 000 - 10 000 m³/s takes only a matter of months, and if undisturbed by further inflows the process of drying up takes one to three years. The annual evaporation rate for the filled Lake has been calculated by Bonython (1955) for 1951 and Tetzlaff and Bye (1978) for 1975 as 2 000 mm: Tetzlaff and Bye also derived a slightly lower value of 1800 mm for 1974. Some indication of the evaporation rate from the dry lake floor (excluding salt crust) can be given by the works of Allison and Barnes (1985), who found a mean value of 170 mm/year for the nearby Lake Frome, and Schmid (1984) who found a value of 100 mm/year for Lake Torrens. Ullman (1985) calculates net evaporation rates from the salt-covered surface of Lake Eyre at between 9 and 28 mm/year; the former is a minimum estimate averaged over some twenty years including the 1974 event, while the latter reflects the most recent summer evaporation period.
However, the high evaporation rate is not the sole reason for the Lake being empty. Lake Titi-caca in South America, of comparable surface area, has an evaporation rate in the order of 1665-1995 mm/year, but remains permanently full. Furthermore, many lakes can be found with higher evaporation rates, even exceeding 3000 mm. In the case of Lake Eyre, the present inflows are simply insufficient to maintain any usable water levels.
Although evaporation decreases marginally as the salinity increases—taking only evaporation into account -maintaining the 1974 water level would require constant inflow of the order of 600 m³/s or 20 m³/year, which is equivalent to the average discharge of the Murray River. For comparison, the Amazon River would fill the Lake to the 1974 level in exactly three days, the Mississippi River in twenty-two days and the Danube River in forty-five days.