DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
2.3.2 Topography and geomorphology
The alluvial deposits form extensive plains with slopes to the west and south of less than one
degree and local relief rarely exceeds a few metres. The relief of the bioregion varies from
40m above sea level (asl) in the southwest to 300m asl in the east. A digital elevation model
(DEM) for the bioregion has been developed using two different levels of resolution (Map
17). The coverage of the eastern third of the bioregion is at a 25 m grid size while the
remainder is at 250 m. It would be preferable if the western portion was mapped at a finer
resolution but this will not occur in the timeframe of this bioregional assessment.
While more detailed modelling is desirable, its value would be limited by the generally low
relief and little variation in elevation within the bioregion. Minor relief may be locally
important (run on and run off areas), but these subtle changes may not be detected even at a
25 m grid resolution. The Narrabri-Lightning Ridge and Collarenebri Interfluve provinces
(see Section 2.6) are the areas most likely to benefit from more detailed modelling.
The present day alluvial landscape of the bioregion is dominated by abandoned quaternary
river systems, which are slightly higher than the modern river systems. Since the Pliocene (2-
5 million years ago) the lower reaches of the tributaries of the Darling system have built
outwash fans from material eroded from the eastern highlands. Watkins and Meakin (1996)
defined four fluvial units in the alluvial landscape of the Nyngan and Walgett sheets: the
Trangie formation, Carrabear formation, Bugwah formation and Marra Creek formation. The
low hill lands adjacent to the riverine plain contain erosional surfaces of pre-Cainozoic rock
blanketed by Quaternary residual and colluvial deposits.
As a part of the Smith et al. (1998) report on the fauna of the northern floodplains, nine
landforms were defined and four of these are present in the DRP. These are:
Alluvial plains and floodplains permanent and temporary streams with channels to 15 m
deep, alluvial plains with minor drainage lines and swamps, some small elevated areas (to
3 m), extensive scalds, some pans, dunes to 10 m, some dunes and rises in channel loops;
Playas and basins small to very large lakes, channels and floodouts, playas (shallow
closed depressions occasionally filled with water), salt lakes, claypans and swamp basins,
feeder channels, levees, lunettes to 10 m, low lying back plains;
Rolling downs and lowlands undulating or low rounded ridges and some higher remnant
hills, broad to narrow drainage lines, rounded pans to 1 km wide, relief to 20 m; and
Plains deposited sediments with many small drainage sinks, broken drainage lines,
floodouts, swamps, floodplains and sandplains, some lunettes, relief to 3 m.
Additional landforms occur in the portion of the DRP outside of the northern floodplains
study area. Some are described within the descriptions of provinces and land units (Section
2.6).
2.3.3 Soils
The soils of the DRP have been mapped at a broad scale resolution (1:2 000 000) by DLWC.
The mapping, current to 1988 (Map 18), shows the spatial arrangement of 10 broad soil
groupings.
Grey, brown and black cracking clay soils cover the 70% of the bioregion in the lower lying
plains and depressions. The areas of higher elevation result from prior stream deposits with
41