Located near the South Australian border, Lake Victoria is a naturally occurring shallow lake about 60 km downstream of the Murray–Darling junction in south-western New South Wales. The lake forms an 'off-river' storage that assists in regulating flow and salinity in the River Murray as it flows into South Australia. It does this in 1 of 2 ways — by intercepting high flows from upstream, or by releasing extra water when required — to ensure South Australia's flow entitlements are met.
In the 1920s, Lake Victoria was modified by the then River Murray Commission to its current state as a regulated off-river storage. Embankments and regulators were built to control water entering the lake from the River Murray via Frenchman's Creek (upstream of Lock 9). These changes increased the maximum capacity of the lake to 677 GL. Water released through the outlet regulator flows from the lake into the Rufus River and returns to the Murray downstream of Lock 7. Lake Victoria covers 12,200 ha, yet only has a maximum depth of about 5.5 m when full.
While the lake is the smallest of the River Murray's 4 major storages, it's location near the end of the system strongly influences the management and level of flows along the entire River Murray.
Lake Victoria is located in a flat, semi-arid region of the Basin and does not have a local catchment of any significance. Its inflows are dependent on diversions from the River Murray. Under natural conditions, the lake would only have received inflows during times of flooding along the Murray or on the rare occasion that sufficiently heavy rain fell at or close to the lake.
The primary function of Lake Victoria is to ensure that South Australia receives its full water share as defined in the Murray–Darling Basin Agreement. Under the agreement, South Australian entitlement varies depending on the time of year. If there is insufficient water arriving from upstream via the River Murray, the flow to South Australia is supplemented with water from Lake Victoria. If there is surplus water arriving from upstream, a portion of the water is diverted and stored in Lake Victoria as long as airspace and inflow constraints allow.
Seasonal variation in operations
Generally, Lake Victoria is filled (where possible) during winter and spring. Depending on tributary inflows and the water level in the lake, filling may involve transferring water from Hume Reservoir to Lake Victoria. This generally occurs during spring through to early summer when there is spare capacity in the River Murray (prior to the main irrigation season), with a flow travel time of about 3 and a half weeks.
Peak demand for water along the Murray typically occurs in late summer. Lake Victoria is located downstream of both major irrigation demands and capacity constraints in the River Murray channel, such as the Barmah Choke. As such, the lake can be used to supply water to South Australia when it is not practical to deliver such water via the full length of the Murray. This is generally during this peak season when upstream irrigation diversions are also high. As a result, the majority of releases from Lake Victoria are made during late summer and early autumn.
In drought, a whole-of-system approach is taken. At the end of irrigation season, this strategy minimises the volume of water stored in Lake Victoria and maximises volumes upstream in the much deeper Hume and Dartmouth Reservoirs. This reduces evaporative losses and the chance of spill from the lake should wetter conditions return.
Coordinating operations with the Menindee Lakes
Since 1988, Lake Victoria has been operated in harmony with the Menindee Lakes. This means that if the flow in the River Murray is too low to maintain a suitable storage volume in Lake Victoria, and there is sufficient water in the Menindee Lakes, water is transferred from Menindee down to Lake Victoria instead of extra water being released from Hume Reservoir. As with transfers from Hume Reservoir, transfers from Menindee are most often made during the period from late spring into summer. In hot weather, storing water in Lake Victoria over the Menindee Lakes is beneficial as less water is lost to evaporation.
Once autumn begins, less water is moved from Menindee to Lake Victoria, maximising the draw on Lake Victoria to meet South Australia's water entitlements. Emptying Lake Victoria like this enables it to be refilled using surplus River Murray flows in the following winter-spring period, which would otherwise spill and be lost if the lake was full of water from Menindee.
The exact amount of water supplied from Menindee and stored in Lake Victoria is dependent upon end-of-month trigger levels in Lake Victoria and whether or not the water is available in the Menindee Lakes.
Lake Victoria is also used to buffer the flow of saline water to South Australia by a process of diversion and dilution. At times when water passing through Lock 9 on the River Murray becomes more saline than the water in Lake Victoria, flow through the lock is minimised which forces more water up Frenchman's Creek and into the lake. The saline water is then diluted by the lake water and flows back into the Murray. Should Lake Victoria be more saline than water at Lock 9, flow through the lock on the River Murray is maximised while releases from the lake's outlet regulator are minimised.
In recent years, Lake Victoria has been used to supplement peak flood flows in the lower Murray, to partially compensate for the impacts of upstream river regulation and water resources development.
During spring in 2000, 2 separate and distinct flood peaks were enhanced to improve the connectivity of the lower Murray floodplain. On each occasion, water was stored in Lake Victoria prior to the arrival of the flood peak, and as the peak arrived, water was released from Lake Victoria to increase the size of the peak. In addition, weir pools — the water stored behind a weir — along the Lower Murray were raised to increase floodplain inundation. This produced a significantly larger area of inundation than would have otherwise occurred.
The spring 2000 events demonstrate the important role that Lake Victoria plays in managing environmental flows in the lower Murray, in addition to its role in securing water resources. Growing awareness of the importance of small-to-moderate floods to the lower Murray floodplain means this style of operation is likely to become more frequent in the future. In planning this type of operation, it is important to consider the environmental objectives of the entire system.
Aboriginal cultural heritage
Lake Victoria has a complex archaeological and historical significance to Aboriginal people and European settlers.
Traditionally, Lake Victoria was a key centre for the Maraura people — a sub-group of the Barkindji. The lake's cultural heritage ensures that it remains important to Aboriginal people today. Artefacts from around the lake indicate a dense pattern of Aboriginal settlement and inhabitation from about 18,000 years ago until the arrival of Europeans in the mid nineteenth century. Stone tools, hearths, grindstones and midden heaps litter the south-eastern shores of the lake.
The sandy areas around Lake Victoria contain Aboriginal burial grounds. However, the extent of the burials was not known until 1994 when the lake was lowered to perform maintenance on the outlet regulator. The sheer number of burials prompted the then Murray–Darling Basin Commission to undertake an archaeological survey. Aboriginal elders were consulted on how best to care for the burial sites and the MDBC initiated protection measures, with local Aboriginal people employed to do much of the work. All of the exposed burials are now protected.
The 1994 study resulted in changes to the way that Lake Victoria is used as a water storage facility. Local stock grazing, as well as artificially high water levels in the 70 years the lake functioned as a reservoir, contributed to the loss of shoreline vegetation and erosion, exposing Aboriginal cultural heritage material. To minimise further such impacts, the MDBC developed a Cultural Landscape Plan of Management (CLPM) in close consultation with local landholders, Aboriginal communities and governments.
Lake Victoria Operating Strategy
Contained within the CLPM is the Lake Victoria Operating Strategy which aims to stabilise the lake foreshore and protect cultural heritage sites by promoting the growth of native vegetation. The periodic lowering of the lake in late summer and early autumn ensures a drying cycle that encourages the establishment of vegetation. As an additional step, when the lake's water level reaches a sensitive height zone on the shore, the water level is altered to minimise the likelihood of wave erosion.
The Lake Victoria Operating Strategy aims to ensure a balance between water users, traditional owners and the environment.