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Fact Sheet 6: The Barmah Choke

Origin of the Barmah-Millewa Forests

Approximately 25 000 years ago, an earth movement in the southern Murray-Darling Basin caused a slight uplift of land and created what is now known as the Cadell Tilt Block (sometimes called the Cadell Fault). The edge of the block runs roughly north/south not far from the towns of Deniliquin and Echuca. Although only about 12 metres high it is an important feature in this otherwise flat landscape. It eventually changed the course, pattern and character of the River Murray for some 500 kilometres.

After the uplift occurred, a large shallow lake was created by the dammed Murray and Goulburn rivers. The Murray soon found a new course around the northern side of the Block now known as the Wakool channel. The river bed it created is today occupied by the Edward River. For thousands of years the Goulburn River continued to feed the lake but it eventually also broke out to the west. Around 8000 years ago the Murray turned south, breaking through the section between Picnic Point and Barmah, and took over the Goulburn channel downstream of Echuca. The section where the Murray cut through to the Goulburn channel is today known as the Barmah Choke because of its limited capacity to carry flows.

During major floods, large volumes of water are temporarily banked up behind the Barmah Choke. This reduces the height of flood peaks downstream, and floods the former lake area. The regular flooding has created a wetland now known as the Barmah-Millewa Forests, the largest area of red gum forest in Australia. These forests contain flora and fauna that would be typical of a region which receives two or three times more rainfall than it does.

 

Peak Water Demand

The period of peak demand downstream of the Barmah Choke is usually late summer or early autumn. Typical flows at this time of year include 7000 megalitres a day (ML/d) to South Australia (but this is usually supplied largely from the Darling or Lake Victoria), 1500 ML/d for Sunraysia, 4500 ML/d for the Torrumbarry irrigation system, and 3000 ML/d for river transmission losses between the Choke and Mildura. Transfers to Lake Victoria and minimum in-stream flows must also be allowed for. The overall total volume that must be supplied daily is thus far in excess of the 8500 ML/d that can be passed through the Barmah Choke without causing flooding. To deal with the difference various management techniques have been developed, and are described below.

 

When the choke causes problems

The Barmah Choke is likely to be a critical factor limiting supply in seasons when:

  • rainfall in the irrigation areas downstream of Echuca is low and irrigation demands are high,
  • Hume and Dartmouth are reasonably full allowing water managers to announce high irrigation allocations; and
  • Menindee Lakes storage is low which means that most of the South Australian share of resources must be supplied from Hume and Lake Victoria.

 

Ways around the Barmah Choke capacity problem

There are various ways of passing water around the Barmah Choke using existing works:

  • Releases from Menindee Lakes on the Darling River are made if MDBC water is available in the lakes.
  • Most of the water requirements of South Australia come from this source in a majority of years.

If it is likely that there will be insufficient supplies in Lake Victoria and the Menindee Lakes to supply South Australia during the coming summer, water may be released early from Hume (typically in July to September) so that it can pass through the Barmah Choke at a time when there is capacity available. It is then stored for later use in Lake Victoria, just upstream from the South Australian border.

Up to 2400 ML/d can be passed via the Mulwala Canal to the Edward-Wakool system and back to the Murray. However this flow capacity is not always fully available for use by the MDBC.

The upper States can be asked to contribute additional flow from their major regulated tributaries, the Goulburn, the Murrumbidgee and the Darling, during periods of peak demand. (these three rivers join the Murray downstream of the Barmah Choke.) However the volume of contributions may be constrained by other requirements on the tributaries.

Water which would normally be released from the Snowy Scheme to the Murray can sometimes be released instead to the Tumut and passed via the Murrumbidgee River to the Murray.

Small volumes can sometimes be passed from the Mulwala Canal to Billabong Creek via the Finley and Wakool escapes.

Small volumes can sometimes be passed through the Yarrawonga Main Channel to the Broken Creek and back into the Murray near the mouth of the Goulburn River.

Flows up to 2500 ML/d can be passed via the Gulf Regulator into the Barmah Forest and via Smith’s Creek and Barmah Lake back to the Murray below the Choke. This route has environmental drawbacks, cuts access in the forest and incurs fairly high water losses, so it is normally used from September to November only, depending on environmental considerations at these times.

 

Impact on water trading

Water trading of water from above to below the Barmah Choke is currently forbidden by both the New South Wales and Victorian Governments because it would increase downriver demand and exacerbate the difficulties caused by the channel constrictions.

 

Past Work on Capacity of the Choke

From time to time there have been efforts to increase the capacity of the river channel through the Barmah Choke. Regulators and block banks have been constructed to prevent unseasonal summer flooding of the forests. Additionally, willows were removed and selective de-snagging was undertaken downstream of Picnic Point in the early 1980s. This resulted in a water level reduction of about 0.4 m at high regulated flows, or an increase in capacity of about 1000 ML/d.

 

Other Capacity Constraints on the Murray

In addition to the limitations caused by the 8500 ML/d capacity of the Barmah Choke other constraints include:

  • channel capacity of the Mitta Mitta between Dartmouth and Hume is about 10 000 ML/d measured at Tallandoon,
  • channel capacity of the Murray between Hume and Yarrawonga is 25 000 ML/d,
  • channel capacity of the lower Darling River 9 000 ML/d,
  • all the major offtakes to irrigation areas,
  • the Edward River and Gulpa Creek offtakes,
  • the inlet and outlet channels to Lake Victoria,
  • release rates from the Menindee Lakes,
  • the supplementary flows available from regulated tributaries such as the Goulburn and the Murrumbidgee. The timing and volume of available releases may also be constrained,
  • The outfall from the Mulwala Canal to the Edward River, and other escapes.
  • Different parts of the system become critical under different seasonal conditions, or under particular storage and demand combinations.

 

The Future

No major engineering works are presently considered necessary. The Barmah Choke is one of several capacity limitations in the system, and should remain manageable with present levels of demand and water transfer requirements downstream.

An investigation in 1997 into progressive changes in the capacity of the Choke concluded that:

  • since the 1970s there appears to have been no systematic change of the river channel cross-sectional area between Tocumwal and Echuca (although local changes particularly near Thistle Bend have occurred); and
  • there is a longer term trend for the flow capacity to increase at the gauges at Barmah (between 1922 and present), Picnic Point (between 1955 and present) and Tocumwal (between 1956 and present).
  • Regular re-surveys and photography should be undertaken each 10 years and after major floods, to monitor trends.

Return to: Review of the operations of Hume and Dartmouth Dams

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