Water was first diverted from the Murray for irrigation in the 1880’s.

The succession of drought years from 1895, culminating in the record dry year of 1902, identified the need to protect Murray Valley residents from drought. This was necessary not only for their livelihood, but also for the continued development of the region.

Construction of the weir began early in 1935, and was completed in 1939 as part of the Murray-Darling Irrigation Scheme.

In 1937 the first suggestions were made concerning the clearing of the red gum forest, to create an open area in the lake. As the River Murray Commission refused to clear the trees, a group of local men took up axes and cross-cut saws, and in 1938 began the enormous task of felling the trees, at the cost of 10 shillings per acre. The River Murray Commission gave no financial assistance to the project, and were concerned that if the felled timber was not burnt or removed, it would prove a hazard to the weir.

The men worked to clear the area, and the tree felling was completed in June 1939. The first filling of the lake was started on 12 August 1939, and many of the felled trees were not removed. The official opening of the weir was scheduled to take place October 27th, 1939. Due to the outbreak of World War II, the weir was not officially opened until October 29th 1989.

Construction began on a stock route on top of the weir early in 1935 and was completed in 1939. The weir has raised the water level in the river so that that the gravity diversion is possible via major channels to irrigate land in both New South Wales and Victoria. A privately owned hydroelectric station and a fish lift were added to Yarrawonga Weir in 1994. The station can generate up to 9.6 megawatts of power. In 2002, Lake Mulwala was drained to allow for earthquake proofing works on the weir bridge. While closed, it was modified to a single lane traffic bridge, although it still remains a stock bridge. Yarrawonga Weir is the largest of the Murray Darling Basin Authority's 16 weirs.

Water impounded by the Yarrawonga Weir forms Lake Mulwala, with a capacity of 117,500 ML covering an area of 4,400 hectares. The full supply level above the river bed is 14.2 meters and 124.9 meters AHD.

A secondary use of the Yarrawonga Weir is flood mitigation. Releases are made downstream before the arrival of flood waters, to help level out the food peak and reduce the flood effects downstream.

Regulating Structures and Embankment

The weir contains two reinforced concrete regulating structures. The design is that of a Regulated Ogee Crest Spillway with a total of ten, 12.2 x 6.1 meters steel gates, attached to concrete counter weights via a cable and pulley sheave assembly. Each gate weighs about 40 tonne. The steel gates are lowered and raised by electric motors located on the steel truss above the structure. 

The weir consists of three sections. The first is the “Southern Regulating Structure” which is 116 meters in length and is located across the river channel. This concrete structure is the main regulator in the weir with 8 vertical lift gates. 

The second and middle section is an earthen embankment separating the regulating structures, consisting of a clay core with steel sheet pilling, covered by selected earth fill shells faced with hand-placed concrete pavers. The earthen embankment extends 270 meters across the river flats, approximately 7 meters in height with an 8 meter crest width.

On the northern side of the weir is the ‘Northern Regulating Structure’, which is 30 meters in length, has 2 steel flood gates and is used to regulate and pass higher flows during floods.

There is another short earthen embankment at the north end of the “Northern Regulating Structure”.

Irrigation

Yarrawonga Weir annual diverts and average of 1,900 GL per year (17% of the river’s average annual flow). The Yarrawonga Weir was built to raise the water level in the Murray River to ensure diversion of water via gravity. Diversion of water is via two major channels, the Mulwala Canal and  the Yarrawonga Main Channel. . The Yarrawonga Main Channel is 957km long and services the Murray Valley irrigation region, from Yarrawonga to Barmah. It supplies water to 128,000 hectares.

Yarrawonga Main Channel

Constructed in the mid-1930s, the Yarrawonga Main channel services the Murray Valley Irrigation Area of Victoria, which extends from Yarrawonga to Barmah, and south to the Broken and Nine Mile Creek systems. The channel is 957km long and has a discharge capacity of 3,100 ML/d and distributes water to an area of 128,000 hectares via a network of 6 main channels and 261 spur channels.

Mulwala Canal

The Mulwala Canal – at 2,880 kilometres – is the longest irrigation canal in the southern hemisphere, spreading across the southern Riverina plain to Deniliquin drawing water from Lake Mulwala. Construction began in March 1935. However, the whole system was not completed until 1942. The Mulwala Canal has an offtake capacity of about 10,000 ML/d and diverts water to the Berrigan, Denimein, Deniboota and Wakool irrigation districts near Deniliquin, New South Wales – a total area of over 700,000 hecteres.

The Canal decreases in capacity downstream to 2,450 ML/d at the Lawson Syphons near Deniliquin, where two large concrete barrels transport water beneath the Edward River and how lying billabongs to supply the Deniboota irrigation district.

Water is also discharged into the Edward River to provide sufficient flow for diversion to the Wakool district via Stevens’s weir.

Management

The Yarrawonga Weir is the largest of the sixteen weirs on the Murray River, and is part of a complex system involved in the regulation of the Murray River and distribution of its waters.

Attempts to manage the Murray River date back to 1863, however interstate rivalry hampered the development of management schemes for forty years. In 1915, the River Murray Waters Agreement was ratified by the Commonwealth and State Governments, and the River Murray Commission was established.

Currently, the Murray River is managed by the Murray-Darling Basin Commission, in close cooperation with the state authorities. The Murray-Darling Basin Agreement was initially signed by the Commonwealth, New South Wales, Victoria and South Australia Governments in 1987, and revised in 1992 and 2012. The fundamental requirement has not changed since 1915 – the MDBA must conserve water, as well as share and supply it to the three States with minimum wastage.

Hydro-Electrical Facilities

The Hydro-electric generation facility was a venture undertaken between the Rural Water Corporation of Victoria and Power Facilities Pty Ltd. Electricity generated by the station is sold under contract to Powercor Australia Ltd. Electricity generated by the station is sold under contract to powercor Australia Ltd. The ownership of the station changed hands in 2001, it is now owned by Meridian Energy a New Zealand, semi Government Corporation.

The station is un-manned and is operated from mount Beauty in Victoria by Southern Hydro, with intricate net-work links to the power station and the Goulburn Murray Water weir office. Construction commenced in December 1992 and power was first generated in June 1994. The stations maximum capacity is around 9 MW and operators with a maximum head differential of 9 meters and flow of up to 12,500 ML per day. 

As part of the project, a new Yarrawonga Main Channel regulating structure was required. The new structure is located 40 meters downstream of the original regulator and incorporates three 6.7 meters wide radial gates giving a total discharge capacity of 3,100 ML/d. Construction of the station and the new Channel off-take took place in the non-irrigation period of May to August 1993 and required draining of Lake Mulwala.

Fishway

A unique feature at Yarrawonga weir is a fish lift, located adjacent to the power station. Constructed in 1994, the fish lift provided the first opportunity in 60 years for fish to travel upstream. Fish are attracted to the power station discharge and into the adjacent fish lift structure. The mechanically operated lift raises fish from the river to the lake level. The cycle of fish attraction, entrapment, raising and release for the lift is automated with a PLC controlled system.

Remedial Works

In 2002, The Murray Darling Basin Commission and Goulburn- Murray Water undertook remedial works to bring this important weir up to modern day dam safety standards. The cost of these works was about $13 million. The main components of these works involved strengthening the foundation of the earth embankment, floodgate piers and the upstream and downstream wing walls of the gate structures. These works to strengthen the foundation of the embankment required stone columns to be installed. “Stone columns’ involve a pattern of compacted gravel columns immediately upstream and downstream of the embankment constructed by compacting gravel into the deep foundation.

The remedial works for the concrete sidewalls consists of strengthening works to the existing walls and installation of reinforcing cables and concrete supports The flood gate piers were strengthened by adding steel reinforcement and mass concrete.

The lake was drained during the non-irrigation period to allow critical components of these works to be completed.

Facts and Figures

General Information

Lake Mulwala

Embankment

Irrigation Channels

Yarrawonga Main Channel

Mulwala Canal

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