Thomson (Th)

	Area: 175 sq. km (0.9%) The lowest levels of the modern flood plains in the main rain-shadow area of the Eastern lowlands, on which clayey alluvium predominates is mapped as Thomson land system. Major river channels, small levees, numerous oxbows, billabongs, swamps and clay plains are included. Stream courses are highly sinuous and meander cutoffs are common. These lowest level flood plains are relatively narrow but they carry the bulk of the flood waters. Traralgon land system also occurs on the lowest levels of the modern flood plains but it is in the more humid parts of the Western lowlands, mostly along tributaries of the major rivers. There has been little soil development on the youthful, mostly clayey and poorly drained alluvia, and most variation in these areas relates to texture and drainage. Some accumulation of organic matter has occurred, particularly in depressions and here organic loams may be found, but commonly fresh alluvium covers darker-coloured, former topsoil. On the well-drained levees textures tend to be lighter and the upper horizons tend to have weak or moderate subangular blocky structures.	A swampy depression on the modern alluvial flood plain of the Thomson River.
Erosion hazards are minimal but many areas are subject to frequent sediment deposition. Clearing of vegetation has facilitated flood flow and increased flooding problems downstream. The native vegetation, probably mainly open forest II of E. tereticornis with Melaleuca ericifolia closed scrub on wetter sites, has been almost completely removed.

CLIMATE Rainfall, mean (mm) Temperature, mean (°C) Seasonal growth limitations	Annual 500 - 800; lowest July (30 - 50), highest October (50 - 80) Annual 12 - 14; lowest July (8 - 10), highest February (19 - 21) Temperature <10°C (av.): June - August Rainfall < potential evapotranspiration: November – March
GEOLOGY Age, lithology	Holocene alluvium
PHYSIOGRAPHY Landscape Elevation range (m) Relative relief (m) Drainage pattern Drainage density (km/km²)	Plains with abundant fluviatile forms on the lowest alluvial terrace 20 - 80 0 - 5 Meander channel 1.8
PRESENT LAND USE	Mostly cleared: grazing of beef and dairy cattle on improved, often irrigated, pastures; cropping (limited); apiculture; recreation — caravaning, fishing and shooting

A study of the land in the Catchment of Gippsland Lakes - Vol 2 - land system Thomson- cs

A study of the land in the Catchment of Gippsland Lakes - Vol 2 - land system Thomson- graph

LAND COMPONENT Percentage Diagnostic	1 45 Plains with many relict channels, oxbows and low-lying areas	² 35 Plains relatively free of channels	3 20 Modern levees, including river channels
PHYSIOGRAPHY Slope %, typical and (range) Slope shape	2, (0 - 5) Straight but some concave	1, (0 - 2) Straight	1, (0 - 2) Variable
SOIL
Parent material	Mostly fine-textured alluvium		Medium-textured alluvium
Description	Soil varies with site drainage — better drained areas with dark brown clay loam to clay; sites with impeded or very poor drainage with dark greyish brown or dark grey clay loam to clay, often mottled throughout; evidence of alluvial stratification common at depth		Levee soils of dark greyish brown loamy sand to sandy clay loam topsoil merging into dark brown or dark yellowish textured subsoil, often stratified
Classification	Wiesenboden, Humic Gleys, Alluvial Soils Dd2.11, Gn2.81, Uf6.11, Um6.21		Alluvial Soils/Brown Earths, Prairie Soil Um6.21, Um6.23
Surface texture	Mostly clay loam, can be much sandier		Loamy sand to sandy clay loam
Surface consistence	Friable to firm when moist, often hard when dry		Very friable to firm when moist
Depth (m)	>2.0		>2.0
Nutrient status	Moderate		Moderate to high
Available soil water capacity	Moderate to high		Moderate
Perviousness to water	Slow to moderate		Moderate to rapid
Drainage	Mostly poor to somewhat poor, some very poor		Good
Exposed stone (%)	0		0
Sampled profile number	-		23
NATIVE VEGETATION Structure of vegetation and characteristic species of dominant stratum (+ Predominant species)	Open forest I, II, often shrubby: Mainly E. tereticornis+, E. polyanthemos+ (drier sites); E. ovata+ and E. viminalis+ (moister sites) Closed scrub of Melaleuca ericifolia+ in wet areas	Grassy open forest II: E. tereticornis+ with or without E. polyanthemos	0pen forest II, III, often shrubby or layered: E. tereticornis+

Disturbance	Affected process and trend	Primary resultant deterioration			Casual activities	Primary off-site process
Disturbance	Affected process and trend	Form	Susceptibility of components	Incidence with components	Casual activities	Primary off-site process
Alteration of vegetation: — reduction in leaf area, rooting depth and/or perenniality	Reduced transpiration, resulting in raised watertable	Waterlogging	1,2; moderate - high	Common in low-lying areas	Reduced plant water-use in the catchment	Increased run-on and ponding in low-lying areas
Increased exposure of surface soil	Increased soil detachment by flood waters	Scour erosion	1; low 2; low - moderate 3; moderate	Uncommon	Cultivating, earth-moving activities	Increased sediment load
Increased physical pressure on soil	Increased compaction With Reduced infiltration	Structure decline Scour erosion	1,2; moderate 3; low – moderate 1; low 2; low - moderate 3; moderate	Uncommon Uncommon	Increased trafficking, cultivation, overgrazing, export of organic matter As for scour erosion above	- Increased run-on and ponding in low-lying areas
Increased soil disruption	Increased soil break-up Increased loosening of sand	Streambank erosion Scour erosion	3; high 1; low 2; low - moderate 3; moderate	Common Uncommon	As for scour erosion above As for scour erosion above	Increased sediment load Increased sediment load
Comments: Streambank erosion is a naturally active process in this land system but has been aggravated by clearing, stream channel modification works, and trafficking by vehicles and stock.

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