Did the River Murray Ever Cross the Mount Lofty Ranges?

A theory as old as the hills – but how old is that, exactly?


The Problem of Morgan

The River Murray descends from the Great Dividing Range at Albury, and for the next 800km follows a fairly steady northwest course.

That changes at Morgan, where the river makes a sharp left turn towards Goolwa and the sea.

The bend at Morgan has been attributed to the Morgan Fault, which lies just west of the river. But if the fault created the bend, where did the river go before the fault was formed?

Did it keep going, and where to?

A western route for the river seems crazy, not least because just west of Morgan lies the impassable barrier of the Mount Lofty Ranges. But a western route was proposed by Williams and Goode in 1978 who searched the ranges for the abandoned river all the way to Spencer Gulf.

The Route Across the Ranges

The western route theory proposes that before the Mount Lofty Ranges were formed, the River Murray continued northwest and discharged to Spencer Gulf. The river was defeated when the ranges were raised and flow was deflected south by the Morgan Fault. Clues to the original route can still be found across the landscape.

The first clue lies in Burra Creek, a small ephemeral stream that drains the western Mount Lofty Ranges and joins the Murray at Morgan. The alignment of the creek is key, because it continues northwest beyond Morgan. Could this be the former channel of the Murray, now flowing in reverse?

The second clue is in the ranges. Burra Creek lines up with the Broughton River, which continues the northwest route on the other side of the range.

The shape of the Broughton River channel is peculiar. Today it is a small, seasonal watercourse – not much more than a creek. But it flows through a channel that is very broad and deep, indicating it was once the course of a major drainage system.

The Broughton River near Spalding

The final clue is in the gulf, where the Broughton River snakes its way across an oversized delta. Although the river small and seasonal, its delta stretches 40 km from Port Pirie to Port Broughton and bulges 18 km into the sea. According to the theory, this is the ancient mouth of the River Murray.

Debunking the Theory

The western route of the River Murray was always hard to believe, but that doesn’t make it easy to disprove. The theory hinged on the River Murray being older than the ranges which, even in 1978, was not clear. It took another 10 years to sort things out.

The Murray Basin formed when Australia separated from Antarctica, 50 million years ago. The sediments that first filled the basin, and have filled it ever since, stop at the foot of the ranges. Therefore ranges have always presented a barrier at the western extent of the Murray Basin.

The sediments washing off the ranges also indicate their age. These date back at least to the Eocene.

As far as the Broughton River delta goes, it’s big, but it isn’t big enough. If you compare it with the vast volumes of sediment carried by the Murray drainage system, the delta is far too small. In fact, the Broughton River delta isn’t much bigger than similar alluvial deposits from the ranges at Noarlunga and Willunga. In the past the Broughton River drained a wetter catchment and would have carried more sediment. And the shallow floor of the gulf contributes to the delta’s broad extent.

In the same way, the oversized channel of the Broughton River is more easily explained by a wetter climate in the geological past than by a westerly flowing River Murray.

So What does the Bend at Morgan Mean?

More recent work has shown that the course of the River Murray through South Australia is actually quite new.

Previously, the Murray Basin drained through the Douglas Depression in western Victoria and reached the sea near Portland. This route was blocked by faulting 2.4 million years ago, which dammed the Basin and created the vast freshwater Lake Bungunnia.

Adapted from McLaren et al. (2011)

The present day route of the Murray formed only 700,000 years ago when Lake Bungunnia was breached. Water spilt southwards from the lake at Swan Reach and carved the Murray Gorge.

Having cut a route downstream, the gorge continued to erode upstream towards Morgan. This route was probably guided by fractures in the underlying limestone that run parallel to the Morgan Fault.

So the deflection of the Murray River at Morgan is an illusion. It doesn’t represent the redirection of a flow moving downstream, but rather the path of the river channel eroding upstream.

References and Further Reading

McLaren, S., Wallace, M.W. and Reynolds, T. (2012). The Late Pleistocene evolution of palaeo megalake Bungunnia, southeastern Australia: A sedimentary record of fluctuating lake dynamics, climate change and the formation of the modern Murray River. Palaeogeography, Palaeoclimatology, Palaeoecology 317-318: 114-127.

Stephenson, A.E. and Brown, C.M. (1989). The ancient Murray River system. BMR Journal of Australian Geology and Geophysics 11: 387-395.

Twidale, C.R, and Bourne, J.A. (2009). Course of the lower River Murray in South Australia: effects of underprinting and neotectonics? Proceedings of the Royal Society of Victoria 121: 207-227.

Williams, G.E. and Goode, A.D.T. (1978). Possible western outlet for an ancient Murray River in South Australia. Search 9: 442-447.


The River Murray Weirs – Part 3 of 3: Mitigating the Impacts

The River Murray Weirs were constructed 100 years ago to provide passage for riverboats and to supply water to inland communities. But the weirs have also degraded wetlands, salinised floodplains and devastated native fish populations. 

In this final article I describe how weirs are now being managed to reduce their impacts and even to help the environment.

Mitigating the Impacts

Over the course of the 20th century River Murray flow was gradually brought under tighter control. Weirs, dams and control systems were used to reduce flow variability and to provide reliable water supply.

Until the 1990s the main purpose of these investments was economic prosperity. But, starting with the weirs, environmental outcomes have gradually been introduced to river operations and environmental health is now central to how the river is run.

1. Fishways

The harm caused by weirs to migrating fish was obvious as soon as they were constructed.

Fish swimming upstream were blocked at the foot of each weir. In 1928, so many fish accumulated below the new Torrumbarry weir that the river was being plundered by commercial and recreational fishers. Fishing was banned for a mile downstream to give the fish a chance. Similar impacts were observed at other the weirs. The weirs represented such a threat to inland fisheries that there was an outcry among fishers and conservationists.

To overcome the problem, fishways were proposed. A fishway is a flowing channel that provides a long, gentle slope for fish to swim around a weir. Fishways had proved effective for salmon in North American rivers, so similar designs were used to build fishways at Lock 6 and Lock 15 in 1930 and 1937.

Unfortunately neither fishway worked well. The designs that suited energetic salmon in fast-flowing snow-melt streams did not suit Australian fish species in the River Murray.

In any case, building fishways in only two of the thirteen weirs was never going to do much good. Due to cost, a lack of ecological expertise and poor cooperation between the states, plans for further fishways were shelved.

It was not until 1990 that an effective fishway, designed specifically for Australian fish, was built into Torrumbarry Weir. This encouraged the installation fishways in all the weirs and the barrages under the Hume to Sea program which has reconnected over 2,200 km of the River Murray from Albury to the Murray Mouth.

2. Floodplain Inundation

The river channel is only a small part of the River Murray ecosystem. By far the biggest component is the floodplain it flows through.

Floodplain inundation at Paringa, South Australia (M. Cooling)

The floodplain includes the forests and wetlands that are inundated when the river breaks its banks. The River Murray floodplain covers hundreds of thousands of hectares and reaches up to 10 km across.

Before the river was developed, the floodplain was inundated almost every year. Flooding has now been reduced by reservoirs that capture high flows. Extractions have also reduced floods by pumping water from the river into farm dams and crops.

The floodplain is now in a state of on-going water stress. Trees are in poor health, vegetation is sparse, wetlands and lakes are frequently dry and frogs and birds have fewer opportunities to breed.

At some key localities, weirs are being used to restore floodplain inundation. Diverting water from the weir pool to the floodplain can replicate a flood event even when river flows are low.

This was first achieved at Lock 8 where water is diverted on to Mulcra Island. Similar schemes have been developed at Torrumbarry Weir and Chowilla and are being planned at Lock 7, Lock 5 and Lock 4.

Weirs are being used to divert water from the river to inundate the floodplain.

3. Water Level Variation

The weirs have been incredibly effective in stabilising river levels. Careful management of the weir enables operators to keep the upstream level steady, even when river flows change.

The weirs are operated to keep upstream levels stable. Downstream levels fluctuate according to flow (MDBA Data).

Stable river levels are harmful to river ecosystem. Wetlands and forests need to be flooded by high water levels in spring. Grasslands and meadows grow on the banks exposed by low levels in summer.

Weirs are now used to provide seasonal cycles in water level. The first seasonal pattern was created in 2012 when Locks 8 and 9 were raised in winter and lowered in summer.

Seasonal weir cycles are now being introduced to all of weirs downstream of Euston.

Lowering the weir during spring allows aquatic plants to colonise the river edge upstream of Lock 8 (Scott Jaensch)

4. Fast Flowing Water

Slow-flowing water has been one of the worst impacts of the weirs.

Weirs make the river deeper and wider. They slow the flow of water to a crawl. Fast-flowing habitat has been virtually eliminated along the continuous chain of weirs from Mildura to the Murray Mouth. Species that depend on fast-flowing water, such as Murray Crayfish and Trout Cod, have gone extinct in the Lower Murray.

There are two locations where fast-flowing water persists in the Lower Murray, and both are floodplain channels that by-pass the weirs. Mullaroo Creek flows around Lock 7 and Chowilla Creek flows around Lock 6. Both support healthy populations of native fish that are absent elsewhere in the region.

There are proposals to create similar conditions by diverting water from weirs into the creeks that flow around Lock 8 and Lock 4.

There are also proposals to restore fast-flowing water in the river channel. If weirs are lowered far enough, fast-flowing conditions can be restored upstream.

There are few places where weirs can be lowered to this extent. Nearly every weir pool has a dense array of irrigation pumps and diversion channels that would be stranded by low water levels. At this stage, Lock 8 is the best candidate.

Opening the weirs lowers the river level and creates the higher velocities on which native fish and crayfish depend.


These are four important ways that weirs are used to improve the environment.

But they do not make up for the damage the weirs do.

Even with fishways, the weirs will never be entirely transparent to fish. Inundating localised floodplains is a poor substitute for natural, widespread flood events. And restoring a few kilometres of fast-flowing habitat is only a token of the former free-flowing 900 km Lower Murray.

The environmental achievements in weir operations are important. But our benchmark should be ‘river health’ and we still have a lot of work to achieve it.

Sources and Further Reading

Barrett, J. and Mallen-Cooper, M. (2006). The Murray River’s ‘Sea to Hume Dam’ fish passage program: progress to date and lessons learned. Ecological Management and Restoration 7: 173-183.

Ecological Associates (2015). Environmental Water Management Plan for the Murray River from Lock 6 to Lock 10 – System Characterisation. Ecological Associates report AL043-1-A prepared for Mallee Catchment Management Authority, Irymple.

Government of South Australia (2012). Weir Pool Manipulation

Lawrence, B.W. (2014). A history of fishway policy on the River Murray – 1928 to 1988. Unpublished article circulated by finterest.com.au


This series of articles is based on work I have done for the New South Wales Office of Water, South Australian Department of Environment, Water and Natural Resources, North Central Catchment Management Authority and the Mallee Catchment Management Authority.

Thanks to Lance Lloyd for comments on the text.

The River Murray Weirs – Part 2 of 3: Eight Ways Weirs Degrade the River Ecosystem

The River Murray Weirs were constructed 100 years ago for riverboat navigation and to supply water to inland communities. But the weirs have also degraded wetlands, salinised floodplains and devastated native fish populations. 

In this second article I describe the impacts of the weirs on the river ecosystem. The final article in this series will describe ecological mitigation and how weirs are being managed for the environment.

How Weirs Work

A weir is a bank built across a river to raise the water level.

Weirs don’t reduce flow: they just create a storage pool upstream. Once the pool is full, water spills over the weir and continues downstream.

The River Murray weirs were designed to support irrigation and navigation, so the pools are kept as stable as possible. Gates in the weirs are opened or closed as flow increases or decreases so that the level in the pool doesn’t change.

The fall across a weir depends on flow. At very low flows a weir pool will be flat and extend to the foot of the next weir upstream (Figure A). Water velocity will be very slow in the weir pool and higher in the tailwater. At higher flows water spilling over the weir raises the downstream level and the river develops a slope (Figure B). The slow-moving weir pool contracts. At very high flows the downstream and upstream levels equalise and the weir is flooded (Figure C).

There are thirteen weirs on the River Murray between Blanchetown and Echuca, and one further diversion weir at Yarrawonga. There are many more weirs on the tributaries.

Eight Ways Weirs Degrade the River Ecosystem

1. Migrating Fish

Golden Perch is a migratory fish species. In winter and spring the fish swim upstream over hundreds or thousands of kilometres to spawn. In upstream reaches, Golden Perch release eggs in response to flood flows. The eggs drift downstream on the floodwater, hatching along the way and drifting into nursery habitats like wetlands.

The weirs are an insurmountable barrier to fish. By blocking their upstream breeding migrations, weirs have played a major role in the decline of Golden Perch, Silver Perch and other migratory fish.

Golden Perch. Fishways now help fish navigate the weirs. Photo Lance Lloyd.

2. Fragmented Habitat

Even if they don’t migrate, aquatic animals need to move around the river.

Species like turtles, Murray Cod, Australian Smelt and Bony Herring move up and down the river to breed, to search for food and colonise new areas.

The movement of these animals is blocked by weirs. The weirs have segmented the Lower Murray into eleven sections, the shortest only twenty nine kilometres long. The weirs reduce the size of breeding populations, reduces access to habitat and makes populations more vulnerable to local threats.

3. Salt

Saline groundwater occurs naturally beneath the Lower Murray floodplain and is widespread upstream of Locks 4, 5 and 6.

When the weirs raised the river level they also brought groundwater closer to the surface. In many areas the roots of floodplain trees – Black Box and Red Gum – were exposed to salt and became sick or died. Groundwater began to seep into wetland depressions. Freshwater plants were replaced by samphire or, in the worst cases, by a crust of salt.

Lower Pike Floodplain at Lyrup, upstream of Lock 4

4. Slow Flow

Weirs have changed the Lower Murray from a flowing river to a series of lakes. The weirs have made the river broad, deep and very, very slow.

Turbulent, flowing water (faster than 0.3 m/s) is essential to Murray Cod, Trout Cod, River Mussel and Murray Crayfish. All of these species are now either extinct or very rare in the Lower Murray.

Turbulent water, flowing through snags, is ideal habitat for Murray Cod and Trout Cod. These conditions are virtually absent from the Lower Murray.

5. Drowned Wetlands

When the weirs were built they raised the river level and drowned low-lying areas. Thousands of wetlands became permanently flooded.

These wetlands used to be flooded seasonally and supported dense beds of wetland plants. The wetlands were breeding sites for fish and waterbirds and contributed to the river food web.

Permanent flooding changed the wetlands into broad expanses of open water. The wetlands are now much less productive and provide poor habitat for fish and birds.

Morgan Wetland, which was drowned by Lock 1.

6. Willows

Willows like the stable water levels provided by weirs and have become a widespread pest plant on the Lower Murray. They crowd the river bank, excluding native trees and reeds, and provide poorer habitat for native fish and birds.

Willows at Morgan, upstream of Lock 1 (@photosbyharriet)

7. Biofilms

Biofilms are the slime of bacteria, fungi and algae that grow on flooded logs and branches. Biofilms are a major food source for snails, small fish and shrimp, which become food for larger animals.

Biofilms are most productive when they are alternately flooded and dried, which happens when river levels rise and fall. Weirs have stabilised river levels, so snags are either flooded or dry most of the time. As a result, an important component of the river food web has been compromised.

Biofilms grow on flooded rocks, snags, twigs and leaves

8. Acid Sulfate Soils

Weirs have created acid sulfate soil risks by waterlogging floodplain soils.

Floodplains soils are often rich in sulfur minerals. When they are waterlogged and become anoxic the sulfur minerals can be reduced to sulfides. If these soils are exposed to the air again, the sulfides are oxidised into sulfuric acid.

The acid is toxic in its own right, but it also liberates poisonous heavy metals from the sediment. Acid sulfate soils can kill fish and plants and make water unsafe to drink.

Acid sulfate soils are safe while they remain flooded. The risk develops when weirs are lowered or when river flows are very low, which occurred in the Millennium Drought (1997-2011).

Catfish Lagoon at Merbein Common, upstream of Lock 10, has potential for soil acidification.


The weirs have made the river less productive and less hospitable to native plants and animals. But few of these impacts can be blamed on weirs alone. Most are made worse by other things, for example:

  • irrigation drainage increases groundwater and salinity threats;
  • water diversions and flow regulation intensify river stability; and
  • carp reduce plant cover in drowned wetlands.

The final article in this series will describe how weirs are now being operated to reduce these threats and improve environmental outcomes.

See Also

The River Murray Weirs Part 1 of 3: Why so many Weirs?

Sources and Further Reading

MDBA (2011). Acid Sulfate Soils in the Murray-Darling Basin. Murray-Darling Basin Authority, Canberra.

MDBC (2007). The Sea to Hume Dam: Restoring Fish Passage in the River Murray. Murray-Darling Basin Commission, Canberra.

Walker, K.F. (2006). Serial weirs, cumulative effects: the Lower River Murray, Australia. Chapter 9 in Kingsford, R. (Ed.) Ecology of Desert Rivers, Cambridge University Press, Cambridge.

Thanks to Lance Lloyd for comments on the text.

The River Murray Weirs – Part 1 of 3: Why so many Weirs?

The River Murray Weirs were constructed 100 years ago for riverboat navigation and to supply water to inland communities. But the weirs have also degraded wetlands, salinised floodplains and devastated fish populations. 

In this article I describe the origin and purpose of the River Murray weirs. In the next articles I will describe their impacts and how they are now being operated to achieve environmental outcomes.


There are thirteen weirs on the River Murray between the Goolwa and Echuca.

Weirs 1 to 11 were built in a continuous series from Blanchetown to Mildura. No more were built until Weir 15 at Euston with another gap to Weir 26 at Torrumbarry.

The weirs were anticipated for fifty years before they were built, but when they were finally completed they were already out of date. Their peculiar numbering and spacing reflects the decline of the river boats, interstate rivalry and the rise of irrigation, road and rail.


Opening the Basin

In the 1850s the Murray Basin was being developed for wheat, wool, sheep and cattle.

The distances from the inland farms to the coastal ports were enormous. Sheep and cattle could be walked to markets, but wheat and wool had to be carted by bullock dray on bad roads over hundreds of miles.

As the only colony with a sea port on the Murray, South Australia wanted to develop the river as a highway to the inland. To start the river trade, in 1854 the government offered a prize for the first boat to reach the Darling River from the river port of Goolwa. Two boats competed and both passed the Darling. And they both returned with cargoes of wool – one from Swan Hill and the other from Echuca 1,570 km upstream.

The Murray Basin was open for business. The river trade boomed and by the 1870s there were hundreds of boats working on the river. Boats travelled inland with food, people and equipment and returned with wool, wheat and timber.

PS Lancashire Lass at Wilcannia towing a barge loaded with wool. State Library of South Australia.

Navigation and Irrigation

River boats were always at the mercy of flow. Traffic would shut down in summer when discharge fell, and drought could strand river boats for years. South Australia wanted to build weirs that would keep river levels high throughout the year, and in 1863 approached the upstream colonies to discuss the proposal.

South Australia had some influence with Victoria and New South Wales while it dominated the river trade. But its position was weakened in 1864 when Victoria completed a rail line to the river port of Echuca and created an upstream alternative to Goolwa.

In fact the upstream colonies were becoming less interested in river transport than in water security. Farmers in Victoria and New South Wales were frustrated by low river flows and lobbied for a reliable supply to protect them from drought and to pursue irrigation.

South Australia looked on anxiously. Irrigation extractions would prolong low-flow periods and make navigation even harder. But worse was the scale of the plans being considered upstream. In 1886 New South Wales and Victoria signed an agreement asserting that all the water upstream of South Australia belonged to them. All of the flow to South Australia was under threat.

Discussions between the colonies were bitter, self-interested and unproductive.

PS Wanera entering Lock 4. State Library of South Australia.

Federation and the Corowa Conference

The 1902 Federation Drought forced the newly federated states to negotiate. Rainfall and river flow were at record lows. The survival of Murray-Darling farms and towns was threatened. Frustrated farmers called a conference at Corowa to discuss water security and dragged state and Commonwealth politicians to the table.

The conference agreed to important principles, including a minimum flow for South Australia and weirs for navigation and water supply. However none of the states were entirely satisfied with the outcome.

In particular, South Australia was unhappy with the low priority given to navigation.

Victoria cannot have been impressed with South Australia’s demands. They had completed a second rail line to Swan Hill in 1890, and the Mildura line was due for completion in 1903. New South Wales had also built lines to Albury, Hay and Bourke between 1881 and 1885, anticipating the demise of the river boats. Neither state was interested in a huge weir infrastructure project of questionable benefit that would sell their produce down the river.

Regardless, South Australia worked urgently to keep the river trade afloat. They passed their own legislation in 1905 to investigate weir sites and in 1910 they passed the Murray Works Act authorising nine navigation and water supply weirs from Blanchetown to Wentworth.

These plans came under a national framework when finally, in 1914, the River Murray Waters Agreement (which became the River Murray Waters Act in 1915) was signed by the states and Commonwealth. This provided for the construction of:

  • a storage on the upper Murray
  • a storage at Lake Victoria
  • twenty six weirs and locks on the River Murray from Blanchetown to Echuca
  • nine weirs and locks in New South Wales, either on the Murrumbidgee or the Darling.

South Australia began work on the first weir at Blanchetown the following year.

The New River

South Australia completed the first nine of the agreed twenty six weirs.

But New South Wales built only four of the remaining structures – Lock 10 at Wentworth, Lock 11 at Mildura, Lock 15 at Euston and Lock 26 at Torrumbarry. Of the nine proposed Murrumbidgee weirs, only two were built and they did not even include locks for riverboats to pass.

Even as the weirs were being completed it was clear that the age of the river boats had ended. In 1934 The River Murray Waters Act was amended to acknowledge that the last thirteen weirs would never be built.

River Murray weirs – elevation and distance from the Murray Mouth.


The struggling river trade was finally killed off after the First World War by the rise of road transport.

But the river trade left a permanent mark on the river. The lower 878 km of the river became impounded in a continuous series of stepped pools.

The weirs primarily became a resource for water supply. The weirs at Torrumbarry, Euston, Mildura and Wentworth each enabled major irrigation developments. South Australia’s weirs were also used to develop extensive irrigation in the Riverland.

The weirs provided a stable and reliable source of water from which to pump. However this has become less important over time, as pumps have improved and upstream storages have become more effective in regulating flow. Several weirs support little or no irrigation.

Next Post

My next post will be about the environmental impacts of weirs. My third and final post on the River Murray weirs will describe how they are now being managed to reduce their impacts and promote ecological outcomes.

Sources and Further Reading

Connell, D. (2007). Water Politics in the Murray-Darling Basin. The Federation Press, Annandale.

Institution of Engineers, Australia (2001). The Engineering Works of the River Murray. Nomination for a National Engineering Landmark on the Centenary of Federation 2001. Institution of Engineers, Australia.

Stagg, H. (2015). Harnessing the River Murray. Helen Stagg, Mildura.

Webster, A. (2017). A colonial history of the River Murray Dispute. Adelaide Law Review, 38: 13-47.

The Gorge, the Lake and the Abandoned Channel of the River Murray


From Overland Corner to Wellington, over three hundred and forty kilometres, the River Murray is carved deep into the landscape exposing thirty metre high pale yellow limestone cliffs.

The gorge is much deeper and wider than today’s relatively small river could create. Its origins lie in a wetter climate, a massive fault block and the palaeo megalake Bungunnia.

The Douglas Depression

In western Victoria an abandoned river connects the Sunset Country to the coast.

Now mostly filled in by sediment, the channel is hard to recognise on the ground. But this river used to drain the Murray-Darling Basin to the sea.

The Douglas Depression is over two hundred kilometres long. Commencing north of Wyperfeld National Park, it runs south to Douglas. It curves west around the resistant sandstone of Mount Arapiles, then continues south-west towards the coast. The depression is up to twenty kilometres wide but shallow – only thirty to fifty metres lower than the surrounding landscape. But it strongly influences present-day drainage systems. The Wimmera River, when it reaches the depression near Natimuk, turns north. The Glenelg River reaches the depression at Harrow and follows it south-west. Along its length are scattered shallow saline wetlands and lakes including Mitre Lake and White Lake in the south and the Wimmera Terminal Lakes in the north: Lake Hindmarsh, Lake Albacutya, Lake Agnes and the Wirrengren Plain.

Oblique Google Earth image of the Douglas Depression looking north from Balmoral.

Below the surface, the depression is carved up to forty metres into the surrounding geology. It cuts through the dune ridges of the Loxton Parilla Sands and deeper into the underlying limestone.

The western shore of Lake Hindmarsh where the Douglas Depression eroded a cliff into the Loxton Parilla Sands. Photo Lance Lloyd.

The depression was created over five million years ago and discharged to the sea near Portland.

Lake Bungunnia

This ancient River Murray was defeated by a tectonic shift.

The Padthway High is a fault block that stretches from the Grampians to the Mount Lofty Ranges, parallel to the coast.

Adapted from McLaren et al. (2011)

The block was raised 2.4 million years ago, raising with it the bed of the Douglas Depression. Flow from the Murray-Darling basin was gradually choked. With no outlet to the sea a vast freshwater lake was created inland.

Lake Bungunnia is the name given to the lake that extended over the lower Murray-Darling Basin between 2.4 million and 700,000 years ago. The lake initially extended from Swan Hill to Swan Reach, north to Menindie and south to Pinnaroo. At its greatest extent the lake covered 90,000 square km. Today’s largest freshwater lake, Lake Superior, is 82,000 square km.

We know the extent of Lake Bungunnia by the erosion of ancient shorelines and the characteristic grey-greenish clay of the lake bed. The clay has since mostly been buried by windblown sands.

But the lake continues to influence ecosystems today: Lake Tyrrell, the Raak Plain and Noora Basin are all formed on the exposed bed of Lake Bungunnia.

The River Murray Gorge

The maximum extent of Lake Bungunnia was determined by the crest of the Padthaway High. The lake reached a maximum extent in the wet climatic period near the start of its life, and peaked at seventy metres above current sea level. At this level the lake just overtopped the fault block at Swan Reach. This was enough to initiate flow to the sea and to start the erosion of a channel for the future River Murray.

Drier climatic conditions followed and the lake level dropped below the sill. But over the next 1.7 million years, during wet periods, the lake periodically overtopped the sill and reactivated and eroded the channel.

Adapted from McLaren et al. (2012)

At this time the sea level was at least one hundred metres lower than today and the coast was far off the shore of today’s Encounter Bay. This provided a steep gradient from the high point at Swan Reach to the river mouth. Lake outflows cut into the limestone of the fault block to create the Murray Gorge.

Oblique Google Earth image of the River Murray gorge looking upstream near Mannum. Vertical exaggeration x 3.

The gorge finally cut down to the level of Lake Bungunnia 700,000 years ago and drained it completely. But the river continued cutting upstream into the bed of the former lake, completing the gorge at Overland Corner.


The ecosystems we see today are the slowly evolving products of geological and geomorphic processes. We have a much better understanding of how ecosystems work and how to manage them when we understand their environmental setting.

See Also

Mallee Dunefields

The Wirrengren Plain

Further Reading and Sources

Bowler, J. M., Kotsonis, A., and Lawrence, C. R. (2006). Environmental evolution of the mallee region, western Murray Basin. Proceedings of the Royal Society of Victoria 118: 161–210.

McLaren, S., Wallace, M.W., Gallagher, S.J., Miranda, J.A., Holdgate, G.R., Gow, L.J., Snowball, I. and Sandgren, P. (2011). Palaeogeographic, climatic and tectonic change in southeastern Australia: the Late Neogene evolution of the Murray Basin. Quaternary Science Reviews 30: 1086-1111.

McLaren, S., Wallace, M.W. and Reynolds, T. (2012). The Late Pleistocene evolution of palaeo megalake Bungunnia, southeastern Australia: A sedimentary record of fluctuating lake dynamics, climate change and the formation of the modern Murray River. Palaeogeography, Palaeoclimatology, Palaeoecology 317-318: 114-127.

Hill, P.J., De Deckker, P., von der Borch, C. and Murray-Wallace, C.V. (2009). Ancestral Murray River on the Lacepede Shelf, southern Australia: Late Quaternary migrations of a major river outlet and strand line development. Australian Journal of Earth Sciences 56: 135-157.

Hallett Headland

This is Hallett Headland looking over Gulf St Vincent in Adelaide. This is the most important bushland reserve in the City of Marion. I recently completed a vegetation survey of council reserves to benchmark progress in weed control and revegetation. Photo Harriet Cooling.

Grey Box

Grey Box (Eucalyptus microcarpa) is a woodland tree that grows in south-eastern Australia, mainly on the lower inland slopes and plains of the Great Dividing Range.

Its main distribution is in northern Victoria and southern New South Wales. There are two isolated populations in South Australia: in the Mount Lofty Ranges and southern Flinders Ranges. In Adelaide, the suburbs of Forestville, Black Forest and Blackwood derive their names from Grey Box.

The tree has distinctive ‘box’ bark: dense, grey and superficially crumbly but tightly held in rectangular tesselations. On older trees the bark becomes dark with larger fragments, like this photo from Reynella.

Grey Box grows up to 20 m, mostly with a single trunk. Woodlands occur mainly where rainfall is between 400 and 700 mm. It is occasionally found the floodplain of the River Murray, notably in Gunbower Forest and Koondrook Forest near Torrumbarry, and in pockets as far downstream as Robinvale. The understorey comprises shrubs and grasses.

Grey Box can occur with Black Box (Eucalyptus largiflorens). The bark of the two trees looks similar, but on the upper branches of Grey Box becomes smooth, coppery and ribbony, but remains grey and tesselated on Black Box.

Grey Box Grassy Woodland is an Endangered Ecological Community under the EPBC Act. It frequently occurs on deep, well-drained loams that are ideal for agriculture that have been preferentially cleared. About 85% of the community has been cleared since settlement.

Mallee Dunefields of the Murray-Darling Basin

Dunefields extend over most of the south-west Murray-Darling Basin.

The dunes and their soils have shaped the vegetation, land use and history of the region. They are wind-blown sands that derive from the underlying Parilla Formation.

Parilla Sand

About 50 million years ago, as Australia was separating from Antarctica, the Murray Basin formed as a broad gulf in south-central Australia. The gulf extended from the Great Divide in Victoria and New South Wales across to the Olary Ranges and Mount Lofty Ranges in South Australia.

After a series of depositional phases, a final marine transgression occurred in the late Pliocene (7.2 to 5.4 million years ago) when the sea reached north of Mildura and east beyond Kerang.

The coastline of the gulf formed an arc that trended north from the Grampians and west to the Mount Lofty Ranges. The Parilla Sand was laid down along this coastline and included near-shore marine sediments and a distinctive coastal barrier dune.

Over the next 2 million years the shoreline periodically moved, stabilised and moved again in an overall retreat to the south-west. In each period of stability, a new coastal barrier dune formed.

This resulted in a continuous series of over 600 parallel ridges. The Parilla Sand is now mostly buried, but many ridges are still visible. Ridges rise from a few metres to more than 20 m and are typically spaced 2-3 km apart. Some individual ridges can be followed over distance of 300 km.

The Parilla Sand is also known as the Loxton-Parilla Sand. It is coarse and gritty, and rich in clay. It is typically pale brown to rusty red-brown in colour.

Parilla Sand on the west shore of Lake Hindmarsh, Victoria (photo Lance Lloyd).

The Parilla Sand rarely outcrops in the mallee. The stranded coastline ridges are most noticeable as periodic hills when travelling east-west along the Dukes Highway or Sturt Highway. A distinctive example is the steep Lawloit Ridge between Kaniva and Nhill.

The vegetation of the ridges is distinctive, supporting Buloke (Allocasuarina leuhmanii), Yellow Gum (Eucalyptus leucoxylon) and Native Pine (Callitris gracilis) in contrast to the adjacent mallee.

Woorinen Sand

After the sea retreated, about 2 million years ago, the Parilla Sand was exposed to wind erosion during arid glacial periods.

Sand from the Parilla Formation was mobilised and reworked with clay from the ancient Lake Bungunnia and limestone from the exposed southern coastline. An extensive wind-blown dunefield developed over most of the western Murray Basin. This is the Woorinen Sand.

The dunes are low with rounded crests that are generally 2-10 m high. They are evenly spaced and linear, running east-west. The soil is orange on the sandy ridges and pale in the limestone-rich swales. The dunes are stabilised by their clay and limestone content.

The east-west dunes of the Woorinen Formation south of Loxton, South Australia. The diagonal outline of the underlying Parilla Sand ridges is visible (Google Maps).

This Woorinen Sand makes up most of the cropping country in the Murray Mallee. It is also the principle soil of the irrigated country along the River Murray west of Swan Hill.

Native vegetation is mainly mallee woodland including Ridge-fruited Mallee (Eucalyptus incrassata) on the dunes and Yorrell (Eucalyptus gracilis) in the swales.

The rounded east-west crest of a Woorinen Sand dune north of Lameroo, South Australia.

Lowan Sand (Molineaux Sand)

More recently, a second dunefield formed in the Murray Basin from erosion of the Parilla Sand.

The Lowan Sand dunefield occurs in three extensive sheets that stretch eastwards from South Australia into Victoria: the Sunset Country from Karoonda, the Big Desert from Meningie and the Little Desert from Mundulla.

The Lowan Sand is pale, glassy, freely draining and mobile. The dunes rise steeply in overlapping parabolas that point to the north-east. Individual dunes can reach more than 40 m high. The sand is white in the south and becomes slightly redder in the north.

The jumbled, parabolic dunes of the Lowan Sands in Ngarkat Conservation Park, South Australia (Google Maps). The white bar is 1 km.

The Lowan Sand seems to originate from erosion at specific elevated localities, where wind deeply stripped the Parilla dunes, resulting in a distinctive nutrient-poor siliceous sand.

The Lowan Sand is called the Molineaux Sand in South Australia. It supports native vegetation adapted to the nutrient-poor, freely draining soil. In the south this is mainly heath, which is a highly diverse community of tough, often small-leaved, low-growing shrubs. In the north the heath has a mallee overstorey, mainly Ridge-fruited Mallee and Scrub Cypress Pine (Callitris verrucosa).

Heath vegetation on Lowan Sand in Billiatt Wilderness Area, South Australia.

The Desert Country

Even by the Indigenous Ngarkat and Wurega people, the Lowan Sand dunefields were sparsely populated.

Settlers characterised this region as uninhabitable ‘desert’ due to the absence of water or feed for horses. Crossing the Ninety Mile Desert was treacherous and avoided by taking routes along the Coorong or Murray.

Attempts to farm the Desert Country consistely failed, despite the region experiencing good rainfall of 300 to 450 mm/year. Soaks were sparse and unreliable. Nutrients were depleted after a few crops. Due to copper deficiencies, stock grazed on the scrub or pasture quickly became sick.

It was not until the 1950s that research into micronutrients in South Australia determined that supplements of copper, zinc and molydenum were the key to opening the region. Development quickly followed, with support from the AMP scheme.

The AMP scheme also promoted development in Victorian mallee. However, development of the Desert Country was mostly postponed. By the 1960s the growing conservation movement successfully argued for its preservation.

The contrasting policies of South Australia and Victoria are evident in the abrupt western edge of several Desert Country reserves along the border.

The Lowan Sand within the Sunset Country at the South Australian / Victorian border (Google Maps)

Further Reading

Bowler, J.M., Kotsonis, A., Lawrence, C.R. (2006). Environmental evolution of the Mallee region, Western Murray Basin. Proceedings of the Royal Society of Victoria 118 (2): 161-210.

Durham, G. (2001). Wyperfeld Australia’s first mallee national park. Friends of Wyperfeld National Park Inc.

McLaren, S., Wallace, M.W., Gallagher, S.J., Miranda, J.A., Holdgate, G.R., Gow, L.J., Snowball, I. and Sandgren, P. (2011). Paleographic, climatic and tectonic change in southeastern Australia: the Late Neogene evolution of the Murray-Darling Basin. Quaternary Science Reviews 30: 1086-1111.

Reuter, D. (2007). Trace element disorders in South Australian agriculture. Unpublished note of the Primary Industries and Resources South Australia, Adelaide.

Tiver, N.S. (1986). Desert conquest: A review of the main events which contributed to the development of the Ninety-Mile Desert. AMP Society, Adelaide.

Thanks to Steve Barnett for reviewing a draft of this article.

Why does the Murray Shrink Downstream of Echuca?

The River Murray channel becomes substantially smaller as it flows downstream of Echuca towards Swan Hill.

The reason is that the Central Murray is an anastomosing system where flow is shared between a number of interconnected, adjacent channels. The three principal channels in this region are the River Murray, the Edward River and Wakool River.

The anastomosing system was initiated by the uplift of the Cadell Fault between 65,000 and 45,000 years ago. The fault obstructed the original westward path of the Murray at Mathoura, forcing water to flow either north or south. The original river channel, now raised 12 m on the uplifted fault block, was stranded and dried out. It remains visible today as a palaeo-channel known as Green Gully.

The River Murray follows the southern path along the foot of the scarp, turning west at Echuca where it is joined by the Goulburn and Campaspe Rivers. The Edward River diverges from the Murray at Mathoura and follows the northern path, reaching the end of the scarp at Deniliquin and resuming a north-westerly course. At this point the Edward and Murray are separated by 55 km with the elevated fault block in between.

The Cadell Fault Block tilts to the west. As the rivers continue downstream, the fault block recedes and Green Gully again becomes the lowest point in the landscape. Wakool River, which branches off the Edward River, adopts the flow path of Green Gully north of Koondrook Forest.

This is significant for the Murray because there is a tendency for the river to resume its former course in the lower-lying country to the north. Breakaway channels divert a proportion of high flows away from the River Murray and towards Wakool River including:

  • Thule Creek via Koondrook-Perricoota Forest
  • Barbers Creek via Koondrook- Perricoota Forest
  • The Little River Murray via Merran Creek
  • Waddy Creek via Merran Creek

These losses result in a substantial reduction in channel capacity in the River Murray between Echuca and Swan Hill. Substantial floodplain inundation is created by sustained flows over 30,000 ML/d at Gunbower Forest but only 17,500 ML/d at Vinifera.

The anastomosing section of the river concludes where the Edward and Wakool rivers converge and then rejoin the River Murray at Kenley. The capacity of the river at this point increases again to 30,000 ML/d.

The contrast between the floodplain systems at Gunbower-Koondrook-Perricoota and Nyah-Vinifera is striking. The downstream sites are miniaturised versions of the upstream systems with a smaller river channel and narrower floodplain. The ecological characteristics are similar with Red Gum forest in low-lying areas and Black Box woodlands at the floodplain periphery.

It is important to use appropriate local hydraulic reference points when interpreting flooding regimes at any point on the River Murray.

The River Murray near Deep Creek (Gunbower Forest). The channel is approximately 80 wide at this point.

The River Murray at Vinifera Forest. The channel is approximately 50 m wide.

The Wirrengren Plain

The Wirrengren Plain is the last lake into which the Wimmera River flows. It is remote – located deep within the Big Desert – and rarely visited. It has been dry for over 140 years.

The Wimmera is a usually a small river, winding through the low-rainfall cropping country of western Victoria. It drains the northern slopes of the Great Divide and the Grampians, flows west through Horsham and turns north at Natimuk towards Dimboola and Jeparit.

In low rainfall years the river dries out after it passes Dimboola. But if the year is wet, the river will flow to the first of the Wimmera’s terminal basins, the 13,500 ha Lake Hindmarsh.

Lake Hindmarsh from The Cliffs (photo L. Lloyd)

Consecutive years of very high rainfall are required for Lake Hindmarsh to fill and for water to flow north through Outlet Creek. After 25 km the creek reaches Lake Albacutya, a 5,500 ha wetland and a Ramsar site. The lake has not been full since 1977, but it was known in the past as an important recreational site with boat ramps and signs marking water skiing areas. It last received inflow in 1996 and has been dry for about 20 years.

Outlet Creek continues northwards from Albacutya and enters the mallee heathland of the Wyperfeld National Park. The creek is enclosed by steep-sided dunes and connects a wetlands that have not been flooded since 1976. While these are the smallest lakes in the system they are still substantial, including Lake Brambuk (150 ha) Lake Brimin (96 ha).

Ten kilometres further north Outlet Creek reaches Lake Agnes, a wide grassy plain surrounded by mallee. The lake has a fringe of Red Gum and Black Box trees which, in this landscape, only occur where there is flooding. Lake Agnes was last flooded in 1918.

Outlet Creek, lined by Black Box and enclosed by dunes in the Big Desert.

The last lake is the most spectacular. The Wirrengren Plain is almost 5,000 ha. When you stand in the centre, the edges are a low dark fringe. In such a dry landscape it is difficult to imagine the volume of water required to fill it, or the lake teaming with fish and supporting thousands of birds. It is an important site to Indigenous people and a known meeting site for people from the Wimmera to the south and the Murray to the north.

The Wirrengren Plain was last flooded in 1874. It was flooded on three occasions in the 19th century, each lasting between 4 and 13 years. The last Red Gums to germinate there are over 140 years old.

Wirrengren Plain with the mallee dunes on the opposite shore in the distance.

Water storage and use have reduced flow in the Wimmera River. But these impacts are minor compared to the enormous rainfall required to generate flow the last of the lakes. The terminal lakes raise questions about climate change, but also the very long cycles under which some ecological systems function.

This post relates to work I did for the Wimmera CMA, Mallee CMA and DELWP.