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| The River Murray in its
natural state had a seasonal cycle of high flows in Spring and low flows in late Summer
and Autumn. In very dry years, the river dried up to a series of waterholes made salty by
groundwater entering them. In flood years water spread out for large distances over the
floodplains (link to: floodplains page), flushing backwaters and billabongs before
draining back into the main stream. In 1917 the River Murray Commission was established to control the rivers flow and ensure that South Australia received guaranteed minimum flows throughout the year. By 1930 six (6) weirs had been built along the river to regulate water flow and help with irrigation and navigation. Locks form part of the weir structures. By 1940 five (5) barrages had also been built at the Murray Mouth. These reduce the amount of seawater moving in and out of the Mouth due to tidal movement and control the water level in the Lower Lakes and River Murray below the first lock and weir at Blanchtown. The building of the barrages allowed the construction of pipelines delivering water from Mannum and Murray Bridge to Adelaide and other towns (called the River Murray Urban Users Region).
These changes have had significant impacts. To look after the health of the River Murray and to ensure future water supplies water that can be diverted from the River Murray for use in Adelaide is currently capped at 650 GL over a five (5) year period (about 130 GL per year). |
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Water Flow and why it's Important It takes a drop of water two months to travel the 2,000 km to the sea down the River Murray from the Hume Dam, near Albury. A drop of water crossing the border into South Australia now takes about a month to reach the sea. The weirs (including locks) and barrages that were built in the Twentieth Century and the removal of water for a variety of uses have resulted in the river flow now being quite different to what it was originally. This has had significant impacts. Have a look at the link below for detail on these impacts. "Excessive quantities of water have been diverted from aquatic habitats by dam building, irrigation and urban water wastage without regard to the maintenance of the biological features that provide many goods and services to society. Altered flooding and drainage patterns have led to the significant loss of habitats and species." and "As one of the Murray-Darling Basin's former Commissioners has stated, the water audit's message is plain: the amount of water presently taken from the rivers is not ecologically sustainable and a new balance between the environmental requirements and the consumptive use will have to be struck (Toyne 1995). This is essential for the long term viability of not only the aquatic ecosystems and rivers, but also virtually all economic activity within the Basin. Murray Darling Basin Commission. To look after the health of the River Murray and to ensure future water supplies water that can be diverted from the River Murray for use in Adelaide is currently capped at 650 GL over a five (5) year period (about 130 GL per year).
Where does the water come from? The River Murray is fed from water and snow falling the Murray-Darling Basin catchment, which covers more than one-seventh of Australia — an area bigger than France and Spain combined. Three-quarters of the run-off in the Murray-Darling Basin comes from the relatively small area of the Snowy Mountains in the Great Dividing Range.
The River Murray in its natural state had a seasonal cycle of high flows in Spring and low flows in late Summer and Autumn. In very dry years, the river dried up to a series of waterholes made salty by groundwater entering them. In flood years water spread out for large distances over the floodplains, flushing backwaters and billabongs before draining back into the main stream. In 1917 the River Murray Commission was established to control the rivers flow and ensure that South Australia received guaranteed minimum flows throughout the year. This put the River Murray Waters Agreement into effect. This was signed by the Governments of Australia, New South Wales, Victoria and South Australia in 1915 after 13 years of negotiations. By 1930 six (6) weirs had been built along the river to regulate water flow and help with irrigation and navigation. Locks form part of the weir structures. The weirs now keep river levels in South Australia fairly constant most of the time. The level rises as more water flows from upstream, due to heavy rainfall in the catchment. Snow melt contributes only a small part of the flow. River levels drop sharply after a flood as water stops flowing from upstream. The rate of fall can be as much as 1-2 metres in a week, due to the combined effect of reduced flows and putting back the weirs. The effect is even more apparent when the demand for irrigation water suddenly increases, because floods usually disappear just as the irrigation season commences.
There are now 11 weirs from Blanchetown to Mildura, controlling the river level like a series of stepped pools. The effect is to ensure that the river never dries up in Summer. Water flows over the top of the weirs when sufficient is available.
The water level can be changed by removing or replacing stop logs, which are concrete bars slid into slots on the weir pillars. During floods, all of the stop logs can be removed. The logs are replaced with very precise timing to ensure that the minimum water level is maintained after the flood recedes. Weirs are usually referred to by the number of the lock chamber which allows boat passage through the weir (ie; Lock 4, Lock 5). However, it is the weir structure that controls water levels in the river, not the lock. Link
A lock is a controlled opening through which boats can pass a weir. When the gates at both ends of the lock chamber are closed, water can enter or be expelled through tunnels in the lock chamber walls. Boats are raised to the upstream level or lowered to the downstream level as required, then the appropriate gate is opened to allow the boat to proceed. It usually takes about 20 to 30 minutes to pass through a lock.
During floods, a special section of the weir can be lowered to the river bed to allow boats free passage without the delay of passing through the lock. This section, called the "navigable pass", is located next to the lock chamber. Link
Large storage dams in the upper part of the catchment hold Spring water run-off for later release to ensure that a minimum amount of water reaches South Australia through the Summer, allowing watering of irrigated crops along the river valley and supplying water to more than three-quarters of the State. Dams of particular relevance to South Australia are: • the Dartmouth Dam on the Mitta Mitta River, • the Hume Dam at the junction of the Mitta Mitta River and the River Murray, and • Lake Victoria, an on-river storage below the Darling junction. Links The importance of this source of water is underlined in drought years, when South Australia's urban use of River Murray water can be as high as 90 per cent of water requirements.
Barrages separate river water from sea water near the Murray Mouth, and stop sea water from flowing up into the river as far as Murray Bridge. As a result, the freshwater river system has been extended to include Lakes Alexandrina and Albert. The barrages are concrete barriers across the mud and sandhills of the Murray estuary. Five (5) barrages stretch from Goolwa to Pelican Point, crossing the Goolwa channel, Mundoo channel, Boundary creek on Hindmarsh Island, and close off Lake Alexandrina with the Ewe Island and Tauwitchere Barrages. The structures include gates which can be opened to let river water out. Tauwitchere Barrage is the longest, with 128 gates. Boats can pass through a hydraulically operated lock chamber in the Goolwa barrage or through a hand operated lock in the Ewe Island Barrage. Interesting links:
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