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Thomson (Th)

A study of the land in the Catchment of Gippsland Lakes - Vol 2 - land system Thomson- geoArea: 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 study of the land in the Catchment of Gippsland Lakes - Vol 2 - land system Thomson- image
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 <10C (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 USEMostly 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- csA 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
    Form
    Susceptibility of components
    Incidence with components
    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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