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
- 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.
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