LAKE

You probably know for its boating, skiing and excellent fishing. What you may not know is how valuable the Lake is to the River Murray System. It plays a part in the lives of irrigators, the community, and and waterbirds downstream.

By managing and varying the level of Lake Mulwala we can provide water at crucial times for irrigators and for the natural environment. This information guide is here to explain the operation of your lake and how important it is to our river system. »How Lake Mulwala »Who operates DID YOU KNOW? was created Yarrawonga weir Until 1969 the maximum operating level in Lake Mulwala was 124.9m. Yarrawonga Weir is operated and In 1939 Yarrawonga Weir was built Additional steel plates were welded across the between maintained by Goulburn-Murray to the top of the weir gates and now Yarrawonga in and Mulwala Water, on behalf of the Murray- the maximum operating level is in to create Lake Darling Basin Commission (MDBC). 125.15m. Mulwala. It is located immediately This operation is directed by the downstream of where the Murray and River Operations team at River »Why Yarrawonga the meet and upstream Murray Water (RMW), the from the Barmah Millewa forest. MDBC’s internal water business, weir was built

Michael Bell which is responsible for operating Yarrawonga Weir was built to raise Lake Mulwala’s storage capacity is and managing the River Murray water levels in order to allow water DID YOU KNOW? system. to flow by gravity into the major Yarrawonga weir was officially around 130,000 Mega litres (ML), or irrigation supply channels Mulwala opened 50 years after it was built. roughly one quarter of the volume of Harbour. Canal (NSW) and Yarrawonga It was completed in 1939, but the Golden perch ceremony was put off due to the (Macquaria ambigua) Main Channel (Vic). Roughly half outbreak of World War II. of the water that is diverted from the River Murray for irrigation is diverted into these large channels. Irrigation season orders for irrigation water supplies and four days, irrigators may request »How the Lake » releases sufficient water from more water. operations Dam to meet the orders. is operated The “official” irrigation season in These greater demands can cause Victoria and NSW Murray System Lake Mulwala to fall slightly. DID YOU KNOW? On the other hand, if conditions are Unlike our regular calendar year, channels usually extends from mid Summer evaporation losses of water from Lake Mulwala is operated by two August to mid May. During the irrigation cooler or it rains in the irrigators’ area, Lake Mulwala are typically 1000 ML per irrigators may use less than was season, Yarrawonga weir is used to main “seasons”: irrigation season day, but they can increase to 2000 ML per ordered. These lower demands can raise water levels to a height range and winter. The operations of the day when temperatures exceed 40˚ C and cause Lake Mulwala to rise slightly. needed to allow water to flow into two seasons are very different and there are strong winds. (The equivalent of depend heavily on weather Yarrawonga Main Channel (Vic) and (NSW). 50mm/day reduction in level of water) conditions and irrigators’ demand for allocated water. The water level in Lake Mulwala will However, it takes about four days for vary within a range of about 124.6 m water released at to reach AHD to 125.15 m AHD for several Lake Mulwala. Within that time weather reasons. During the irrigation season can change, and if conditions are River Murray Water assesses daily warmer than predicted over the last Between hume and Lake Mulwala

ays tra 4 d vel tim Map Legend e Nature Reserve/Park yarrawonga Forest Reserve LAKE MULWALA Water body (e.g. Lake) HUME DAM Townships Yarrawonga Watercourses Weir Wodonga Travel time

DID YOU KNOW? RMW directs the operation of the River Murray System to share the water amongst NSW, Victoria and South .

downstream), any extra releases down- Mulwala to allow maintenance on the »“Rain rejections” stream from Yarrawonga Weir will cause Therefore, RMW tries to prevent weir below normal lake levels or for the water to flow into the Barmah-Millewa unseasonal flooding of Barmah-Millewa storage of floodwaters. Even the best weather forecasters find it Forest. as far as possible. The lower the difficult to predict thunderstorms. When water level in Lake Mulwala when a rain Flood events usually occur during the rainfall is greater than previously This forest is the largest River Red Gum rejection occurs, the more water can be winter operating season, meaning Lake forecast, there can be a very sudden fall forest in Australia and is recognised as stored in the lake minimising Operators must ensure that floodwaters in irrigation demand. However, the water a of international significance unseasonal flooding of the forest. from the Ovens, Kiewa and Murray ordered over the previous four days is under the Ramsar Convention. However this level must remain high Rivers are safely passed downstream. already on its way from Hume Dam. enough to maintain irrigation supplies. This usually involves having the lake This drop in irrigation demand because Under natural conditions the forest of rain on irrigators’ properties is called at low levels to provide a measure of would generally be wet in winter-spring operational flexibility, particularly during a “rain rejection”. but dry during summer. Excessive »Winter operations high flows. It means that the extra water will either summer flooding of Barmah-Millewa be stored in Lake Mulwala, causing Forest, which may be due to rain The winter operating season for Lake the water levels to rise, or be released rejections, can alter the fragile balance Mulwala is generally from mid May to downstream. As the downstream river of conditions needed for the vegetation, early August. It is during this period that channel is full during most of the as well as native animal populations, to it may be necessary to draw down Lake irrigation season (supplying irrigators remain healthy. » Glossary

River Murray System Snag Australian Height Datum (AHD) The main course of the River Murray Fallen trees and dead branches in the A datum is a known or assumed point and anabranches including tributaries river channel. used as a basis for inference or reckoning. entering the River Murray upstream In navigation and surveying, the mean of Albury and the Mega litre (ML) sea-level is a common datum-line. AHD, downstream of the Menindee Lake A Mega litre is 1,000,000 litres and is the Australian Height Datum was adopted storage. This includes the Edward roughly equal to the volume of an in May 1971, and is the datum to which River System and it also includes Olympic swimming pool. all mapping is referenced in Australia. dams and weirs such as Hume and This datum surface passes through a Dartmouth Dams Yarrawonga Weir. carefully calculated mean sea level.¹ Ramsar Convention The broad aim of the Convention on Weir Wetlands (Ramsar, Iran 1971) is to halt A dam placed across a river or canal to the worldwide loss of wetlands and to raise or divert the water or to regulate conserve those that remain through wise or measure the flow of the water. Under use and management. The Convention DID YOU KNOW? high flow conditions most of the crest was signed by representatives at the town The dead snags found in Lake Mulwala are excellent habitat of a weir is overtopped and downstream of Ramsar in 1971. There are now more for and other fish and birds. Lake Mulwala is one water level is only marginally lower than 135 Contracting Parties to the of Australia’s premier Murray cod recreational fisheries and an than upstream water level. Convention who have designated the important breeding ground for the species. Ramsar List of Wetlands of International Importance.

Lake Mulwala is an extremely important part of the River Murray System. The ability to regulate flows four days travel time downstream of Hume allows very efficient use of water by irrigators supplied from Lake Mulwala. This also means water can be effectively managed to minimise impact to properties, the environment and to the river channel itself.

For more information on how your Lake is essential to the River Murray System please call the Murray-Darling Basin Commission on (02) 6279 0100, or visit the website: www.mdbc.gov.au

¹ Reference from (http://e-nrims.dwlbc.sa.gov.au/a1pgs/glossary1.htm) Photos, unless otherwise specified, by Arthur Mostead. Graphics/illustrations adapted from Arthur Mostead, Gunther Schmida and Michael Bell. Graphics by Toni Brasting.