Nature conservation

Biodiversity Reform

Murray Darling Depression - landform

Topography

The Murray Darling Depression Bioregion lies in the Murray Basin on Tertiary and Quaternary sediments deposited from a shallow sea, lakes and rivers. The bioregion extends into Vic and SA.

The landscape is characterised by dunefields, sandplains and undulating plains of brown calcareous soils. There is very little structured drainage but numerous lakes, swamps and depressions are present, some of which are driven by saline groundwater.

Geology and geomorphology

The Murray Basin is a shallow crustal depression filled with marine and terrestrial sediments to a maximum depth of 600m over the last 50-60 million years. Shallow seas have moved back and forth across the plains several times, leaving traces of parallel beach ridges and limestone sediments under the dunefields. At one stage the coast reached as far inland as Balranald.

Sandy surface sediments have been extensively reworked into dunes and sandplains that have blown onto the Cobar Peneplain. Some dunes have consistent east-west linear patterns, others are parabolic, suggesting differences in vegetation cover, sand supply or age.

The Darling River and streams in the Riverina have cut through the sands and constructed numerous overflow lakes such as the Sayers Lake system and the abandoned Pleistocene channels and basins of the Willandra Lakes complex. Saline groundwaters have formed salt basins in many places where the sandplain or dune topography intersects the water table.

All lakes and swamps have well-formed lunettes on their eastern margins that record evidence of climate change and human occupation. A few bedrock ridges rise above the sandplains as isolated ranges.

Geodiversity

The Murray Darling Depression Bioregion has many important wetlands. Other significant geodiversity features include:

  • flooding frequency varies and water quality and lake or swamp environments are very diverse;
  • abandoned systems, such as the Willandra Lakes, preserve evidence of past climates and environments along with abundant archaeology; there are many equivalent lunette sites that have not yet been examined;
  • heavy sand mineral resources and large deposits of gypsum are known but not exploited.

Soils

Soils and vegetation differ according to the landform. On the dunefields red, brown and yellow calcareous sands occur with more clayey materials in the swales. On sandplains the soil tends to be heavier with brown gradational or texture contrast profiles, and mallee is found only on sandy rises.

Lakes and depressions all have clay floors. The more saline lakes have grey cracking clays and carry chenopods. Salt lake floors carry little vegetation. Lunettes comprise varying soils from clean sands, brown clayey sands, mixed sand to clay.

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Page last updated: 26 April 2016