Lake Mulwala & the Yarrawonga Weir
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.
A stock route was built on top of the weir in 1939. 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.
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 Mulwala Canal is 2,880 kilometres long and is the largest irrigation canal in the southern hemisphere, spreading across the southern Riverina plain to Deniliquin and suppling water to 700,000 hectares. 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.
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.
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.
- Storage capacity = 117,500 mega litres (1/4 of Sydney Harbour)
- Area = 4,450 hectares
- Length = 489 metres
- Distance from Murray source = 528 km
- Distance from Murray mouth = 1,992 km
- Full supply above river bed = 14.2 metres
- Full supply above sea level = 124.9 metres
For further information see “Harnessed Waters – A River Damned” ed. Noel Loughnan