Soil/Landform Unit 166

The entrenched valley slopes and alluvial floor of the Curdies River

Area:

14 834 ha

1.11% of CMA region

Dissection of lava plains and Neogene sediments in the south-west of the Western Plains of Corangamite has lead to the development of a steeply dissected entrenched valley with a broad alluvial floor. This unique landscape includes the Curdies River and minor contributing streams. The entrenched valley and valley floor includes Herb-rich Foothill Forest, Riparian Forest and Lowland Forest as the major vegetation classes. This dissection has produced steeply dissected terrain highlighting a wide diversity of geologies (Neogene marl, limestone, sands and gravels, and Quaternary volcanics and alluvium) in these steep slopes and scarps. The unit consists of black cracking clays on the alluvial plains. On the steep slopes of the entrenched valley, acidic mottled texture contrast soils, shallow brown clays and black and grey cracking clays are dominant. On the upper slopes and crests black texture contrast soils and acidic mottled texture contrast soils occur. Land use includes water supply, grazing, cropping, horticulture and nature conservation. Erosion hazards are high on the scarps and steep slopes, which have a high susceptibility to soil loss. Landslip and rock falls are also hazards of this steep terrain. Areas with shallow stony gradational soils are susceptible to leaching of nutrients, and compaction is a problem on the alluvial plains.

Major dissection of the remnant plain has left slopes that drain into stream south of Cobden
	Drainage line of the Heytesbury with prominent vegetation lining the stream banks

Component	1	2	3
Proportion of soil-landform unit	10%	60%	30%
CLIMATE Rainfall (mm)	Annual: 850
Temperature (^oC)	Minimum 9, Maximum 19
Precipitation less than potential evapotranspiration	October-March
GEOLOGY Age and lithology	Neogene fluvial-marine sediments, minor marl, calcarenite and Quaternary basalt
Geomorphology	6.2.2 Dissected plains, rises and low hills of the Sedimentary Western Plains and 6.2.4 Plains, rises and low hills of the Sedimentary Western Plains
LANDUSE	Uncleared: Nature conservation; passive recreation; water supply Cleared: Dairy farming; beef cattle and sheep grazing; cropping; water supply (stock); passive recreation
TOPOGRAPHY Landscape	Undulating alluvial plains and valley slopes
Elevation range (m)	1-169
Local relief (m)	40
Drainage pattern	Rectangular-parallel
Drainage density (km/km²)	2.8
Landform	Entrenched alluvial valleys
Landform element	Rise and crest	Side slopes	Drainage channel
Slope and range (%)	4 (1-7)	8 (5-15)	1 (0-3)
Slope shape	Convex	Convex	Concave
NATIVE VEGETATION Ecological Vegetation Class	Herb-rich Foothill Forest (6.1%); Riparian Forest (2.6%); Lowland Forest (1.1%)
SOIL Parent material	In situ rock/sediments, colluvium; sand, silt and clay.	Colluvium and alluvium; clay, silt and sand	Alluvium; clay, silt and sand
Description (Corangamite Soil Group)	Black texture contrast soils (32), mottled brown, grey or yellow texture contrast soils (13)	Acidic mottled brown, grey or yellow texture contrast soils (15), shallow brown clays, black and grey cracking clays (24)	Black cracking clays (31)
Soil type sites	CLRA8, SW42, OTR492	CLRA9, OTR499, OTR784, SW63	SW37, CLRA48, CLRA15
Surface texture	Fine sandy loam	Fine sandy loam, light clay	Medium clay
Permeability	Moderate	Moderate to low	Low
Depth (m)	<2	<1.5	>2
LAND CHARACTERISTICS, POTENTIAL AND LIMITATIONS	Texture contrast with surface soils having low to moderate water holding capacity and nutrient holding capacity, higher in subsoil. Moderately rapid site drainage, surface soils have moderate susceptibility to sheet and rill erosion and nutrient decline. Variable soils and soil depth mean variable susceptibles. Subsoils may have expansive clays.	Texture contrast surface soils have low to moderate water holding capacity and nutrient holding capacity, higher in subsoil. Rapid site drainage, slope and topographical position mean surface soils susceptible to sheet and rill erosion and nutrient decline. Variable soils and soil depth mean variable susceptibilities.	Heavy expansive clays, permeable when dry, very poor permeability when wet and susceptible to compaction. High nutrient holding capacity. Very poor to poor site drainage.

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