The Story of Yarrawonga Weir and Lake Mulwala

Irrigation Begins on the Murray

The first diversion of water from the Murray River for irrigation began in the 1880s, a modest start to what would become one of Australia’s most extensive water management systems. The need for greater water security became urgent during a series of severe droughts from 1895, culminating in the devastating drought of 1902. These challenges underscored the necessity of a reliable irrigation scheme—not only to protect livelihoods, but also to ensure the sustainable development of the Murray Valley region.

Constructing the Yarrawonga Weir

In response, construction of the Yarrawonga Weir began in early 1935 as part of the broader Murray-Darling Irrigation Scheme, and was completed in 1939. Built to raise the river level and enable gravity-fed water diversion, the weir helped irrigate large areas in both Victoria and New South Wales.

Simultaneously, work began on a stock route across the top of the weir, completed alongside the weir in 1939. This route remains in use today, although it now accommodates single-lane vehicle traffic.

Clearing the Red Gum Forest

In 1937, proposals emerged to clear the red gum forest that would be submerged by the newly created lake. Although the River Murray Commission declined to undertake or fund the work, local initiative prevailed. In 1938, a determined group of men—armed with axes and cross-cut saws—began the enormous task of felling the trees for 10 shillings per acre, receiving no financial support. The Commission worried that if the timber was not removed or burned, it might become a hazard.

By June 1939, the felling was complete. On 12 August 1939, water began to fill the newly formed Lake Mulwala, although many felled trees were left in place. The official opening of the weir was delayed due to the outbreak of World War II, and did not take place until 29 October 1989, exactly 50 years after its completion.

Lake Mulwala: Capacity and Function

Water held back by the Yarrawonga Weir forms Lake Mulwala, a body of water spanning 4,400 hectares and holding 117,500 megalitres at full capacity. The full supply level sits at 14.2 metres above the riverbed or 124.9 metres AHD.

While its primary function is irrigation, the weir also plays a critical role in flood mitigation. Water is strategically released downstream in advance of floodwaters to reduce peak flow and minimize downstream impact.

The Weir's Structure and Operation

The Yarrawonga Weir is the largest of the Murray-Darling Basin Authority’s 16 weirs and is composed of three main sections:

1. Southern Regulating Structure
   Spanning 116 metres across the river, this reinforced concrete structure features eight vertical lift gates, each weighing approximately 40 tonnes. The design uses a regulated ogee crest spillway, with gates operated via electric motors linked to a cable and pulley system.

2. Earthen Embankment
   Separating the regulating structures is a 270-metre-long earthen embankment, about 7 metres high, with a clay core reinforced by steel sheet piling. The outer structure is protected by hand-placed concrete pavers.

3. Northern Regulating Structure
   At 30 metres in length, this structure contains two steel floodgates for controlling high flows during floods. A smaller embankment extends beyond this structure at the northern end.

Irrigation Infrastructure

The Yarrawonga Weir facilitates the diversion of around 1,900 GL of water annually—about 17% of the river’s average annual flow—via two major channels:

Yarrawonga Main Channel

Constructed in the mid-1930s, this 957-kilometre channel services the Murray Valley Irrigation Area in Victoria, extending from Yarrawonga to Barmah and reaching as far south as the Broken and Nine Mile Creek systems. The channel has a discharge capacity of 3,100 ML/day, delivering water to 128,000 hectares through six main channels and 261 spur channels.

Mulwala Canal

At 2,880 kilometres, the Mulwala Canal is the longest irrigation canal in the southern hemisphere. It draws water from Lake Mulwala and supplies the Berrigan, Denimein, Deniboota, and Wakool irrigation districts, servicing over 700,000 hectares. Construction began in 1935 and was completed by 1942.

A key engineering feature is the Lawson Syphons near Deniliquin, where water flows through large concrete barrels beneath the Edward River, supplying the Deniboota area and allowing diversion to the Wakool district via Stevens Weir.

Modern Management and Governance

Efforts to regulate the Murray River began as early as 1863, but political friction between the states stalled progress. Finally, in 1915, the River Murray Waters Agreement was ratified, and the River Murray Commission was formed.

Today, management is overseen by the Murray-Darling Basin Authority (MDBA), established through agreements signed in 1987, and revised in 1992 and 2012. The core objective remains unchanged: to conserve, share, and efficiently distribute water among the participating states.

Hydroelectric Power and Fishway

A significant upgrade came in 1994, with the addition of a hydroelectric power station and fish lift adjacent to the weir. Developed in partnership between the Rural Water Corporation of Victoria and Power Facilities Pty Ltd, the facility was sold in 2001 to Meridian Energy, a New Zealand semi-government agency.

The power station, capable of generating 9.6 megawatts, operates unmanned, controlled remotely from Mount Beauty by Southern Hydro. It was built during the non-irrigation period of 1993, requiring Lake Mulwala to be drained.

At the same time, a new regulating structure for the Yarrawonga Main Channel was installed, located 40 metres downstream of the original. Featuring three radial gates, it has a discharge capacity of 3,100 ML/day.

The fish lift, constructed in tandem with the power station, was the first in over 60 years to allow fish to travel upstream. Fish are drawn to the power station discharge and lifted via a mechanical system that automatically cycles through attraction, entrapment, elevation, and release.

Remedial Works and Earthquake-Proofing

In 2002, a major upgrade was undertaken to bring the weir in line with modern safety standards. Managed by the Murray-Darling Basin Commission and Goulburn-Murray Water, the $13 million project involved:

• Reinforcing the embankment foundation using compacted stone columns.

• Strengthening floodgate piers, wing walls, and concrete sidewalls with new supports and reinforcing cables.

• Upgrading flood gate structures with steel reinforcements and mass concrete.

Once again, Lake Mulwala was drained during the non-irrigation season, allowing these critical works to be safely completed.

Facts and Figures

General Information

Lake Mulwala

Embankment

Irrigation Channels

Yarrawonga Main Channel

Mulwala Canal

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