5.3.3 Salt lake depression (Douglas Depression)

This third tier sub-division is made up of one large continuous unit located in Western Victoria; one of the most southerly occurrences of the Dunefields and Plains geomorphological Division (Tier 1; 5).

The northern boundary is with the Little Desert (unit 5.2) and the Wimmera River (unit 4.1) whose broad valley forms a continuing depression link to depression units further north. The Douglas Depression is at its widest here, about 8-9 km across, decreasing in width further south. To the west of the depression and to some extent to the east lies a complex of ridges and depressions, the ridges often subdued (unit 5.5.2). The degree and clarity of incision of the Douglas Depression in the landscape varies, probably most defined by its deflection around Mount Arapiles (unit 5.6), an inlier of the Grampians sandstone complex. The depression is dominated by a series (chain) of lakes, of which most are saline. The depression width varies from 2-3 km near Natimuk and 4 -5 km to the south of Douglas where larger lakes (e.g. White Lake) occur. The depression surfically peeters out (after about 85 km from the Wimmera River) to the south of White Lake on encountering the Dundas Tableland (unit 2.3), which rises gently to the south before being dissected by the Glenelg River.

Extending north-east of Harrow to the Little Desert, the shallow sinuous north-sloping valley comprises a suite of landforms including gentle valley inclines, almost level plains, backplains and lunettes associated with salt and freshwater lakes. This depositional system is characterised by gentle slopes that reflect reliefs of 10 m. Lakes here are significantly larger than lakes further north within the Lowan Salt Valley. Drainage is poorly defined with no through system recognised.

The Douglas Depression is an area of saline discharge, as evidenced by the lake systems (often with lunette formations) and varying periods of water present, and is likely to be structually controlled. The depth and age of regolithic material in the depression is not well known. Regolith is typically alluvial sourced including significant deposits of clay and gypsum that are strongly associated with salinas and lunettes during arid dune-building phases (Bowler 1976; Hill & Bowler 1995). It should be noted that similar formations of stranded beach ridges occur on both sides of the Douglas Depression (the ridges were formed approximately 4 Ma). There is an incursion of Lowan Sand across the depression to the north of Douglas, somewhat masking the depression and the older ridge material. There is speculation, given the sharp flow direction change of the Wimmera River that the depression is a paleochannel of a south to north flowing stream or indeed a north-south stream before the uplift (doming) of the Dundas Tableland (unit 2.3); some form of river capture has taken place. Another theory is that this was an outlet for a former lake (Lake Bungunnia) as the inland sea of the Murray Basin retreated.

The depression is an extension of what is known as the Lowan Salt Valley. The climate becomes milder and moister south of the Little Desert thus the calcareous soils and mallee vegetation start to disappear southwards; the Lowan Salt Valley is the most southerly incursion of mallee type environment into the riverine environment further south.

The Douglas Depression has a range of soils from calcareous earths (Calcarosols) in the north to sodic and saline texture contrast and cracking clay soils to the south and amongst the lakes and swamps. These texture contrast soils and heavy variants are prone to seasonal waterlogging with very poor site drainage. While sandy soils (Kandosols) can occur as aeolian sand spill overs from the western plains, most soils are extremely sodic at depth and often saline when in association with salinas.

Vegetation communities include a number of woodland and wetland associations including Heathy Woodland, Plains Woodland, Low Rises Woodland, Riverine Chenopod Woodland, Seasonally Inundated Shrubby Woodland, Lunette Woodland, Plains Woodland and Salt Paperbark Woodland. Other vegetation communities include Aquatic Herbland, Plains Savannah, Ridged Plains Mallee and Dunefield Heathland. The depressions are also associated with a number of vegetation communities that grow in saline conditions including Lignum Swampy Woodland, Brackish Sedgeland, Red Gum Wetland, Plains Savannah, Brackish Wetland, Inland Saltmarsh, Samphire Shrubland and Saline Lake Mosaic, as well as permanent and semi-permanent saline wetlands.

Soil-landform unit	Unit description	Area (km²)
Douglas lake and lunettes	Lake and lunette	58
Natimuk-Douglas valley	Shallow valley	293

Figure 16 Lakes and lunettes (White Lake and Centre Lake) at Douglas

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