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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.

Conclusion

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

Acknowledgements

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.

Conclusion

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.