7.2 South eastern riverine plains

7. Eastern Plains (EP)

7.2.1 Floodplains and morasses (Powlett, Tarwin, Moe, Latrobe, Thomson, Avon, Mitchell, Jack and Tarra Rivers, Dowd's Morass)
7.2.2 Prior stream plains (Agnes, Yarram, Yinnar, Tinamba, Clydebank)
7.2.3 Older alluvial plains (Stratford, Briagolong)
7.2.4 Plains with dunes (west of the Perry River)

The Gippsland Riverine Plains dominate the region north of the La Trobe River and south of the Eastern Uplands between Traralgon and Bairnsdale. Smaller areas are recognised around Yarram, Yinnar and south-west Gippland near Toora. Also included are the present flood plains and morasses. The plains are of alluvial origin, with the most of the alluvium being derived from the Eastern Uplands, and, in south Gippsland, the Southern Uplands. The alluvium which comprises the surface material is mostly Quaternary, but over the period of its deposition, all of the stream courses which deposited the alluvium are not evident, apart from the most recent.

Throughout the Quaternary have been several considerable rises and falls in sea level. It is generally believed that this was because of changes in volume of the ice caps due to climate changes. Alternatively sea level changes may have been associated with uplift of the land surface. A number of distinct areas of similar elevation, or terraces were formed during the Quaternary in Gippsland, but whether they were formed as a result of sea level change associated with ice ages or by uplift or a combination of both is uncertain.

During the last Glacial Period about 17 000 to 20 000 years ago a sea level fall of about 150 m resulted and a land bridge was formed between Australia and Tasmania. As well as being cold, the climate was dry and windy. The Latrobe, Macalister, Thompson, Avon and Tarra Rivers cut deep valleys into their earlier flood plains as a result of the sea level fall which subsequently became partly in-filled as the sea level rose to its present level. This has resulted in a well defined break between the old flood plain (upper terrace), and the present flood plain (lower terrace).

Six terraces are recognisable within the Gippsland Riverine Plains, but it is more convenient here to aggregate them here into three geomorphological units: present flood plains and morasses; prior stream plains; and older alluvial plains and terraces. The second oldest terrace is the most extensive, extending from just east of almost to Bairnsdale. This terrace, which is partly mantled with dunes, as well as the oldest terrace, is described under Section 7.3 “High level terrace and fans” .

The inland sand sheets and dunes on the plains east of Stratford were earlier interpreted as being sand barriers and foredunes of marine origin and the boundaries of the terraces regarded as stranded shorelines. Their presence at elevations up to 128 m above sea level, far higher than if the level of the sea would reach if all of the world’s ice were to melt, was attributed to continuous uplift of the land and sea level changes associated with past glacial periods. It is now believed that most of the dunes are younger than the terraces upon which they lie and are terrestrial in origin.

Further reading

Aldrick J. M. at al. 1988. A Study of the Land in the Catchment of the Gippsland Lakes Department of Conservation Forest and Lands, Land Protection Division. Victoria, Australia.

Hill S. M. and Bowler J. M. 1995. Linear dunes at Wilsons Promontory and south-east Gippsland, Victoria: relict landforms from periods of past aridity. Proceedings of the Royal Society of Victoria 107, 73-81.

Jenkin J. J.1968. The geomorphology and Upper Cainozoic geology of south-east Gippsland, Victoria.. Memoirs of the Geological Survey of Victoria. 27.

Vandenberg A. H. M. & O’Shea P. J. 1981. Explanatory notes on the Bairnsdale 1:250 000 geological map. Report of the Geological Survey of Victoria. 65.

Ward W.T. 1977. Geomorphology and Soils of the Stratford-Bairnsdale area, East Gippsland. Victoria. C.S.I.R.O Soils and Land Use Series No 57.

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