Australia: The Land Where Time Began
The riverine plains of southeastern Australia are divided into eastern and western sections. About 200 km west from the eastern margin of the riverine plain the undulating grasslands merge with the more arid western section, with its gypsum playas, each usually associated with a lunette. About 100 km further west there is a sharp change, at the boundary of the Ivanhoe Block, to longitudinal sand dunes trending east-west. The natural boundary is so abrupt it shows up on Landsat images. On the ground the change of vegetation type is also obvious, from the saltbush of the western Riverine Plain to the mallee of the Ivanhoe Block.
The groundwater system of the Darling is separated from that of the Lachlan-Murrumbidgee by the regional divide of the western ridge of the Ivanhoe Block. In the deeper aquifers the flow is directed south by the parallel basement ridges of the Ivanhoe Block. Because groundwater flow is blocked by the Ivanhoe Block, the eastern Murray Basin in New South Wales discharges its groundwater into the groundwater discharge zone of the western Riverine Plain. The Riverine Plains are divided into 2 groundwater salinity zones. An eastern zone of the eastern Riverine Plain, less saline than the western zone, where the risk of salinisation of non-irrigated land is low and a western zone of the western Riverine Plain, where even on non-irrigated land is much higher, as the groundwater is very salty. The risk of salinisation is high in the area around the discharge zone and the lower Willandra Lakes, an area where watertables are rising. In between is a buffer zone where the risk of salinisation is intermediate.
The salinisation problem would be greatly reduced if the alluvial fans of the Lachlan and Murrumbidgee Rivers could flow into the Darling River, as they would if they weren't blocked by the Ivanhoe Block. Yet another part of Australia that wasn't designed for agriculture.
Apart from an extension to the southeast, between the Goulburn River and the Loddon River, in Victoria, most of the Riverine Plain is in New South Wales. It has been found that the Riverine Plain is composed of alluvium derived from the southeast highlands of New South Wales, which was spread by the rivers flowing to the west from the Great Divide and to the north from the southeastern highlands in Victoria, over an area of about 77,000 km2.
The modern rivers, Murray River, Murrumbidgee River and the Lachlan River, meander across the plain, which makes them very different from the palaeorivers and streams with sandy beds that spread the alluvium across the plain by wide networks of distributaries. Among them were also meandering rivers similar to the modern rivers, but with a water volume that is believed to be about 5 times the flow of the modern rivers.
Dating of these sediments has shown them to have been deposited over a period of about 100,000 years, being of ancient and modern alluvium and lake deposits, with some aeolian additions. The only tectonic activity of the area of the Riverine Plain was the activity associated with the uplift of the Cadell Block along a fault line between Deniliquin and Echuca, that changed the flow patterns of the ancestral Murray River, that occurred about 30,000 years ago. It has been found by dating sediments of Thule Lagoon in Green Gully that the tectonic activity began about 60,000 years ago. The major river bend south of Echuca formed when the Murray diverted south of the block through the Barmah sandhills, after diverting north along the channel of the Edward river for a short time, as recently as 10,000 years ago.
A period of major river flow on the Riverine Plain of New South Wales is evidenced by the widespread sediments deposited at that time. A corresponding pluvial period deposited widespread sand sheets in the Lake Eyre Basin, between 120,000 and 95,000 BP. The flow declined between about100,000 and 85,000 BP. Between about 50,000 and 40,000 BP there is another period of increased stream activity in the northern section of the plain, that corresponds to increased activity elsewhere, in Wester New South Wales in the Wilandra Lakes as well as other inland playa lakes, and in Victoria, Lake Tyrell in the northwest, and in Queensland, in a more limited active phase in the western River and the Gilbert River.
This was followed by a transitional phase in which the rivers became more sinuous with mixed loads, and flows much higher than the present rivers. Near Darlington Point, the ancestral Murrumbidgee is believed to have had as flow about 5 times that of the present. Between Narrandera and Carrathool, and along Yanco Creek, on the edges of the Murrumbidgee floodplain, there are long sections of these meandering palaeorivers that have been well-preserved.
It has been found that there were 4 palaeodrainage systems in the palaeochannels of the Murrumbidgee River, of decreasing age correlating to changing climatic regimes. These 4 systems are:
Coleambally palaeochannel system
This is the oldest of the palaeochannels, it is in the area of the Coleambally Irrigation Area. A single channel left the western edge of the Yanco Creek floodplain near Morundah and branched into 2 channels, both of which have low sinuosity and low levees. The sediments of these channels date from about 100,000 to 85,000 BP.
The northern channels, above and below the modern river, form a complex system, a southwest trending arm of which crosses the centre of the region. Near Moulamein this channel is covered by modern sediments of the Edward River. It is believed this system was active prior to about 50,000 BP, which corresponds the a subpluvial phase in southern and inland Australia when river activity was high, and Lake Urana, situated on the plain, was in a lake-full phase.
Gum Creek system
This system is a meander belt upstream of the Yarrada Lagoon that has cut between 1 and 3 m below the plain surface. It controls the present course of the Murrumbidgee River. The greater discharge of the palaeochannel compared to the modern river is indicated by the longer wavelength of the meanders. Dates have been determined from between 35,000 and 20,000 BP. It corresponds to the sinuous phase of the ancestral Murrumbidgee River.
Yanco palaeochannel system
About 15 km west of Narrandera this system branches south from the modern Murrumbidgee River, trending southwest and west towards Moulamein across the plain to terminate abruptly at the margin of the present Eduard River floodplain. The present channels and floodplain of Yanco Creek and Billabong Creek occupy the northern part of the system, where they have cut about 2 m into the plain. Downstream from Wanganella, in the southwestern section, the palaeochannels are not used by modern streams. The width of the palaeochannel has an average width here of 225 m. At various times between 20,000 and 13,000 BP it must have been a major river. During the last glacial phase of the Pleistocene ice age its peak flows correlate with ice melts on the catchments.
By about 12,000 BP the Murrumbidgee River had reached its present flow regime, and now has major urban salinity problems in the upper catchment in towns such as Wagga Wagga and Yass.
Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|