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Darling Riverine Plains - landform

Topography

The Darling Riverine Plains Bioregion occupies most of the upper catchments of the Darling and Barwon Rivers in northern NSW and southern Qld and includes the channels and floodplains of the lower reaches of these catchments.

The upper catchment landscape is a series of overlapping, low gradient alluvial fans. The lower tract of the river is a narrow floodplain confined between bedrock landscapes, or by extensive sandplains and dunefields.

Discharge from past and present streams control patterns of sediment deposition, soils, landscapes and vegetation. Much of the geology and geomorphology of the region is similar to that of the Riverina Bioregion.

Geology and geomorphology

The main streams contributing water and sediment to the alluvial fans of the plains are the Bogan, Macquarie, Castlereagh, Namoi, Gwydir, Macintyre, Narran, Bokhara and Culgoa rivers. Sheets of alluvium up to 100m thick have been deposited on older sedimentary rocks and contain marine sediments of an inland sea of Cretaceous age.

Almost all bedrock features have been buried in this sedimentary basin, with only a few high points of basement rocks such as Mt Foster rising above the plain, and more extensive areas of the Cretaceous sandstones forming low rises around Lightning Ridge and in the Collarenebri interfluve.

Tributary streams below Bourke are ephemeral and contribute little water or sediment to the main stream. Downstream of Wilcannia the Darling breaks into several channels (anabranches) that flow on roughly parallel courses for up to 200 km before joining the Murray.

The Darling River is subject to extreme flow variation. River discharge declines downstream as water is lost through seepage and evaporation. The upper margins of the plain, especially in sandy soils, are part of the recharge area of the Great Australian Basin. The river may have zero discharge for several consecutive months, alternating with regional floods that may last nearly 12 months.

Overall the landscape is flat with river channel and floodplain features dominant. Not all of the region has been effectively mapped but in those areas where detail is available, such as Nyngan-Walgett, the complexities of geomorphology and surface sediment distribution all reflect past climates and different river discharge regimes.

Each main stream carries different sediments depending on catchment geology and rainfall. The Bogan, for example, rarely flows strongly and only carries suspended clay past Nyngan. In contrast, the Castlereagh floods more often and carries a sand bed load because it drains extensive areas of Jurassic sandstones with higher rainfall.

The Namoi deposits clays derived from volcanic rock so the floodplains below Narrabri are some of the most productive soils in the state.

The Macquarie is the largest tributary and has the most complex alluvial fan. This river drains a large area of the South Eastern Highlands Bioregion, and because its headwaters are in the high rainfall zones of the Great Dividing Range to the east it has a high discharge, is subject to large floods, and is sensitive to climate change.

Any long-term change in average discharge causes the Macquarie River to change form and shifts the location of sand or clay deposition on the alluvial fans.

Between Bourke and Wilcannia the confined Darling River channel has a simpler landscape of channel, floodplains, billabongs and slightly higher red soil terraces. Below Wilcannia the stream breaks into anabranches and is often attached to large circular or ovoid overflow lakes, which can be up to 15 km in diameter but are only a few m deep.

Three types of wetland are found in the bioregion: delta-like swamplands, terminal drainage basins and lakes, and overflow lakes filled by floodwaters that drain back to the river as the flood recedes. The Macquarie Marshes are the most important and extensive example of a throughflow delta-like swamp in the bioregion.

Narran Lakes are an example of terminal basins at the end of the Narran River, a distributary channel of the Balonne. The Menindee Lakes complex are the overflow lakes.

All lake beds consist of grey cracking clays and the eastern margins of most lakes have well-formed sandy beaches and crescent-shaped dunes or lunettes up to 25 m high which are composed of fine cemented quartz sand with some layers of pelleted clay.

Geodiversity

Important features of this bioregion include the following:

  • the wetlands of this bioregion are the most important wetland habitats in the inland regions of the state;
  • the Gwydir raft below Moree is a classic example of a giant debris dam that is now causing the main stream to change course across the Gwydir fan;
  • Lightning Ridge, also within the bioregion, is the only commercial black opal field in the world;
  • the entire bioregion is an important example of an inland drainage system where the streams flow into an arid region; the catchment has a long geological history and contains numerous sites that have the potential to yield information about past climates, past environments and human pre-history; only a few of these have been examined and they have become important heritage sites as a consequence; examples are Cuddies Springs and the lunette of Lake Tandou.

Soils

Soils and vegetation directly reflect past patterns of sedimentation and today's flooding regime, with some variation in plant species across the region relating to summer or winter rainfall dominance.

Sandy soils are found in linear belts along the older stream channels, sometimes with local source dunes on their border. Texture contrast soils, often badly eroded, are found marginal to channels of all ages, and most of the plains are dominated by deposits of heavy dark-coloured clays. Many clay areas have gilgai micro-relief patterns, most crack extensively, and others are more or less permanently wet in swamplands.

The sandy soils have low nutrient levels and drain rapidly. The clay soils vary more depending on source rocks in the catchment, but all have only a limited amount of free water available to plants. Most soils contain high levels of calcium carbonate and some are saline.

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Page last updated: 26 April 2016