Terraces Animation Transcript

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Scene one

Imagine a time many hundreds of thousands of years ago a stream flowing in a broad valley several kilometers wide.

Scene two

With each flood, successive layers of sediment derived from erosion of the mountains and hills in the upper catchment are deposited. These sediments are called alluvial sediments and the resulting plain is called a flood plain.

Scene three

A period occurs when sea levels fall due to earth movements or shifts in global climate (such as a glacial period), or both, which increases stream gradients and hence the erosive power of streams. thiss results in erosion of the former flood plain until the stream reaches a new gradient such that it can no longer erode downwards.

Scene four

This results in the creation of a new valley within the old (former) flood plain. A new stream and flood plain becomes established within the valley. The remnant of former flood plain above the level of the new stream is called an alluvial terrace.

Scene five

Following global climate change and the partially melting of the polar ice caps, the sea level gradually rises. Climate changes may also increase the rates of erosion of the mountains and hills in the upper catchment, further adding to the rate of alluvial deposition in the valleys.

Scene six

After a period of stability a shift in global climate period and sea level again occurs which increases stream gradient and hence the erosive power of the stream. Again erosion occurs and an even newer valley and stream becomes established.

Scene seven

After a prolonged period of low sea levels, another global climate change occurs and the sea levels rise. Again there is a change in stream gradient and the valley refills with alluvium resulting from erosion higher in the catchment.

Scene eight

This animation shows the formation of (only) two alluvial terraces and a flood plain. Similar cycles of prolonged glacial period periods followed by warmer periods and sea level rise have occurred many times in the past. Now there are five clearly defined alluvial terraces as well as the present flood plains of the major rivers between Traralgon and Bairnsdale in Gippsland.

Scene nine

There are distinct, observable soil profiles.

Scene ten

Soils on the older terraces are older and show more profile development, i.e. clay movement down the soil profile resulting in strong texture contrast between surface and subsoils. Subsoils are generally well developed, coarsely structured (e.g. coarse prismatic or columnar) and invariably sodic. In this soil profile almost all of the clay has been leached from the surface soil horizons and redeposited in the clay subsoil. It is a strong texture contrast soil with very weakly structured loamy sand sharply overlying yellowish brown medium to heavy clay subsoil. The strongly bleached subsurface (A2) horizon indicates that periodic waterlogging occurs above the slowly permeable clay subsoil.

Scene eleven

Soil profiles on the intermediate terraces show less profile development, i.e. less clay movement down the soil profile, resulting in moderate texture contrast between surface and subsoils which show moderate structure development and may or may not be sodic. In this profile, much of the clay has been leached from the surface horizons and redeposited in the clay horizon that occurs from 60 cm. It is a texture contrast soil with weakly structured silty to fine sandy loam overlying a moderately structured reddish-brown light to medium clay. The lack of a strongly bleached subsurface and the red/brown colours indicate that this soil is rarely waterlogged.

Scene twelve

Soil profiles on the present flood plain show little to no profile development, i.e. no clay movement down the soil profile resulting in more uniform textures throughout the profile. Strongly developed subsoils are not present and but can show some structure development depending on the clay content and are generally non sodic. This soil profile is uniformly textured with weakly structured silty loams to silty clay loams throughout. Apart from slightly darker colour in the surface soil, due to organic matter, uniform colours and soil textures occur throughout the soil profile. There may, however, be slight variations due to separate flood events.

Scene thirteen

Where water is available, the lower terraces and flood plains are irrigated and support productive pastures and increasingly the flood plains are used for vegetables. (This is likely to continue until the next major shift in climate).

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