Running the river

Each day, River Murray Water staff 'run the river' by deciding on releases from storages along the River Murray and lower Darling. We release water to meet the needs of irrigators and flows for South Australia within constraints such as minimum flow requirements, dilution of salinity, maximum rates of change of water level, and capacity of the river channels. Instructions on releases are then given to staff of the State Authorities located at the various river structures.

An overview of how releases are determined for major water storages is provided below.

Data required

Each day, more than 350 items of data are received from points along the River Murray and tributaries to facilitate operation of the River Murray system. This data comes from agencies in New South Wales, Victoria and South Australia. The split between various types of data is provided in the chart below.

This data is put into a computer database as well as manually recorded on 'operations sheets'. A computer based model uses data from the last few weeks to predict flows at key points along the River System. The model takes into account the time it takes for water to flow between points, the magnitude of diversions of tributary inflows (River Murray Water obtains forecasts from the States), irrigation water, 'loss' of water due to evaporation and seepage, and the change in volume of storages and weir pools. The model also estimates future requirements for water.

After consideration of downstream demands, losses and tributary inputs, releases are determined for each major storage to ensure efficient delivery of water to downstream users. Staff at the river structures are then instructed to implement the releases.

The three modes of running the river

This is a summary of considerations in calculating the amount of water to release from major storages for each of the three 'modes' of river - supplying mode, storing mode, spilling mode. The major storages are Dartmouth Dam, Hume Dam, Menindee Lakes and Lake Victoria.

It is possible for one reach of the system to be run in one mode, while another reach is simultaneously run in a different mode. For example, a major flood in the Darling River in January may lead to spilling mode being employed for the Darling River and the River Murray downstream of Wentworth, while at the same time the River Murray upstream of the Darling Junction may be operated in supplying mode to meet irrigation and other demands.

1. Supplying mode

Supplying mode occurs for most of the irrigation season, which typically extends from the start of November each year until mid May. During supplying mode operations, the flow in the river is set to meet the demands (including the entitlement flow to South Australia) with little excess. Operation in supplying mode includes the following considerations:

setting releases from each storage to meet downstream water supply requirements, river transmission losses and dilution flow requirements;
making use of tributary flows;
setting releases from Lake Victoria to supply South Australia's entitlement flow;
transferring water from the Menindee Lakes to Lake Victoria;
transferring water from Dartmouth Reservoir to Hume Reservoir;
limiting flows to within channel capacity, which varies along the river system;
observing maximum rates of rise and fall of river levels set to minimise bank slumping and other problems;
maintaining minimum flows at key points in the river system;
manipulating airspace at upstream storages, while maintenance works are underway, so as to temporarily reduce flows at downstream storages; and
making releases for environmental purposes, such as maintaining flows to wetlands for waterbird breeding.

2. Storing mode

The storing mode of river operation generally occurs during the winter and spring seasons, when the flows in the river are in excess of that required to meet diversions, water supply, and minimum flow requirements; but which are confined within the channel. When the river is operated in storing mode the majority of water originates from high flows in the tributaries, particularly the Ovens, Kiewa and Goulburn rivers.

Operation in storing mode includes the following considerations:

capturing surplus flows in Lake Victoria for subsequent release during supplying mode;
making minimum releases from each storage;
operation of forest regulators for forest watering purposes;
monitoring tributary inflows, and advising stakeholders if there is a possibility of channel capacity being exceeded; and
pre-releasing (ie releasing prior to a flood to create airspace) from Dartmouth Reservoir, Hume Reservoir and Menindee Lakes at rates up to the downstream channel capacity.

3. Spilling mode

Spilling mode occurs when flow exceeds the river's channel capacity at a point as a result of runoff generated by heavy rain. This operation can be quite complex as the flow varies as tributaries join the main stream. The channel capacity varies along the River Murray. For example, the channel capacity:

downstream of Dartmouth Dam is approximately 10 000 ML/day
between Hume Dam and Lake Mulwala is approximately 25 000 ML/day,
immediately downstream of Yarrawonga Weir is approximately 60 000 ML/day, and
at the Barmah choke is about 8 500 ML/day.

Floods may be generated from tributary flows or flood releases from storages, or a combination of these.

Spilling mode occurs at a storage when there is limited 'airspace' left in the storage and inflow rates are high. Operation in spilling mode typically occurs in late winter and spring.

Spilling mode includes the following considerations:

pre-releasing (ie releasing prior to a flood to create airspace) from Dartmouth Reservoir, Hume Reservoir and Menindee Lakes at rates up to the downstream channel capacity;
regularly adjusting flow control gates at Hume Dam and Yarrawonga Weir to attempt to mitigate the peak of a large flood;
where possible, limiting releases from upstream regulated sources (including the Snowy Mountains Scheme and Dartmouth Reservoir);
assisting the Bureau of Meteorology with providing flood warnings and forecasts of peak flood levels downstream of dams;
removal of weirs 1-11 and 15 to allow the flood unimpeded passage;
operation of forest regulators for forest watering purposes;
releasing flushing flows from the Barrages (at the mouth of the River Murray) to help clear sand banks and sand bars;
using Lake Victoria to attempt to mitigate the peak of a large flood; and
enhancing flood peaks and durations downstream of Lake Victoria to improve the environmental outcomes of small floods when there are no adverse impacts on the community.