Riverina - landform
The Riverina covers the alluvial fans of the Lachlan, Murrumbidgee and Murray Rivers west of the Great Dividing Range and extends down the Murray. Much of the geology and geomorphology of the region is similar to that of the Darling Riverine Plains Bioregion.
The upper catchment landscape is a series of overlapping, low gradient alluvial fans. The lower tract of the river is a floodplain with overflow lakes. Discharge from past and present streams control patterns of sediment deposition, soils, landscapes and vegetation.
Geology and geomorphology
This bioregion is dominated by river channels, floodplains, backplains, swamps, lakes and lunettes that are all of Quaternary age. The region comprises three overlapping alluvial fans centred on the eastern half of the Murray Basin. Features of each fan differ slightly because of differences in the discharge of the streams.
The Lachlan fan is mainly clay as this smaller stream does not have the competence to carry sand. The other two fans are similar except that the Murray is more confined and has more active anabranch channels where it is forced to flow around the obstacle of the Cadell fault near Echuca. At times of extreme flood flow, water from the different streams can cross the fan surfaces and enter channels of another system.
The evolutionary story of these fans is one of decreasing discharge through time that parallels the story of the Darling Riverine Plain. Different phases of stream discharge have been linked to prior and ancestral stream patterns each with different form and different bed load characteristics.
Between 15-30,000 years ago, prior streams carried sand far onto the clay plains in wide channels with long meanders. Ancestral streams are more like the modern channels, with tighter meanders and carrying only clay. Alluvial sediments become deeper and older in the western half of the basin, reaching a maximum thickness of about 500m. Basement rocks are the early Palaeozoic sediments and granites of the Lachlan Fold belt, but almost no outcrops exist in the Riverina.
As in the Darling Riverine Plains, three types of wetland are found: delta-like swamplands particularly on the Lachlan, terminal drainage basins and lakes on Willandra Creek and overflow lakes on the Murray.
Lake beds are covered by grey cracking clays and the eastern margins of most lakes have well-formed sandy beaches and crescent-shaped dunes or lunettes up to 25m high and composed of fine cemented quartz sand with some layers of pelleted clay. Older, abandoned lakes are also widespread.
Important features of this bioregion include the following:
- evidence of changed Quaternary environments and human history is preserved in the landscape patterns of prior streams, lake beds and lunettes;
- coarse sand in prior stream beds is an important economic resource in a region without rock outcrop; and
- the wetlands of this bioregion are very important habitats in southern inland regions of the state.
Modern river channels consist mostly of sandy soils and more saline heavy grey and brown clays towards the outer perimeter of the floodplains on the higher rarely flooded terraces (Eardley 1999). Sandy soils also form levees, old channels, dunes and lunettes.
As soil and water salinity increase downstream on the Murrumbidgee, saline clays become evident on lake floors.
The red-brown and grey clays in the bioregion support grassland communities that are nationally significant. Calcareous, sandy soils, that tend to be feature of adjacent bioregions are also present in the Riverina and support mallee communities (Semple 1990, Porteners 1993, cited in Eardley 1999).
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Page last updated: 27 February 2011