Australia: The Land Where Time Began

A biography of the Australian continent 

Ross River

This river has a 1570 km2 catchment in the eastern Macdonnell Ranges. It is the largest single tributary of the Todd River. It rises in the high ranges on the Proterozoic Arunta Block, flowing through the lower ranges and hills onto the wide, flat plains, flowing south to the Todd. The Ross River crosses its floodplain as a well-defined channel about 300 m wide, and up to 5 m deep in the sediment deposits, on its way to the Todd River. In its northern section it is a single channel meandering across the plain, but by the southern reaches it forms braided channels by large diamond-shaped bars.

Alluvial sediments underlie the floodplain. The associated palaeochannels are visible on aerial photos. Contact between a major palaeochannel about 1000 m wide and the adjacent aeolian dunefield marks the western margin of the floodplain. The wide palaeochannel swings to the south-east, becoming part of the Todd River floodplain. Upstream of the confluence of the Todd and Ross Rivers are sandsheets, undisturbed by floods during the Holocene, that obscure palaeochannels of the Todd.

The alluvial deposits and palaeochannels of the Ross River have preserved a record of the palaeofloods. The records of Holocene floods, their frequency and the area covered by them, can be found in the stratigraphy of the floodplain. Beneath the floodplain are alluvial deposits characterised by red earth soils dated to more than 59,000 BP. These sediments are so deeply weathered that there is no evidence of their alluvial history. A sheet deposit of very silty sand, of probable aeolian origin, from 9200 +/- 900 BP, formed in the Late Pleistocene arid period, covers this alluvium. This arid period is thought to correspond to the Younger Dryas event. Deposits in the Gulf of Carpentaria dating from 11,375-10,430 BP, records increased aeolian activity at this time. This is associated with the 25,000 year sunspot activity cycle.

The oldest alluvium from the Holocene can be found in low bars and levee deposits beside the present channel. Also as low, long-wavelength bedforms fanning out across the plain. This deposit was laid down by a 10-km-wide flood that covered the entire plain. It has been dated to less than 10,000 BP. There is also a 1500-500 m wide palaeochannel that records several large flood between 1500 and 700 BP. The present narrow, deep channels of the river in the older alluvium of the plain is the latest phase of the river's evolution.

The pattern of long periods of stability, with a few intervening large floods, seem to be concentrated in the Late Holocene, as has been found with the Finke River.

Sources & Further reading

  1. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
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                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading