Snowy Montane Rivers Increased Flows

High-flow events are releases of 1,400 ML/d or greater. This is the flow threshold at which the department and Safety Technical Advisory Committee consider that potential safety risks may begin to occur, based on local knowledge and evidence from past releases. For flows of 1,400 ML/d or greater, the department instigates the processes set out in the Safety Management Plan including extensive public communications.  
The threshold has been incrementally increased from 1,000 ML/d to 1,300 ML/day and now to 1,400ML/d as evidence from past releases has been collected.

Changes to the definition of high-flow over recent years mean that less releases are classified as ‘high-flow” events for safety management purposes. Large flows are still being made (i.e. between 1,000 ML/d and 1,400 ML/d).

The volume and timing of high-flow environmental flow releases (in excess of 1,400 ML/d) for the upcoming water year are listed on the department’s website. All daily release volumes are listed on Snowy Hydro’s website.

Each high-flow event will commence at 12pm (midday) local time for a 24-hour period.

For the high-flow releases, the days leading up to and following the release will have intermediate flows to create a more natural flow release pattern (hydrograph) that maximises ecological benefits.

The water will be released into the upper Murrumbidgee River from Tantangara Dam and travel down the river, through the ACT and continue until it enters Lake Burrinjuck, NSW.

In addition to the high-flow releases, frequent moderate and smaller releases are made throughout the year. These are listed on Snowy Hydro’s website.

As an outcome of the Snowy Water Licence Review, flexible delivery of smaller flows was introduced last year on a trial basis, whereby the date of some releases may be changed to respond to natural cues such as rainfall events. This trial will continue throughout 2025/26 to help support river health and water dependant plants and animals including native fish.

This adaptive management approach is commonly applied in other rivers’ environmental watering programs.

This means that for releases under 1,400 ML/d, the volume and timing of planned releases may change without notice. Flexible delivery will not apply to high-flow events (i.e. greater than 1,400 ML/day) for the 2025/26 water year.

Delivering flows in response to natural cues aims to give dam releases a better chance to combine with rainfall-driven tributary inflows across the upper Murrumbidgee River catchment. This assists healthy river health and water dependant plants and animals including native fish.

Flexible delivery will not apply to high-flow events (i.e. greater than 1,400 ML/d) for the 2025/26 water year, although we may move towards greater flexibility of high-flow events in the future. Appropriate safety management planning would be developed to support this.

There will be periods throughout the year where no environmental flows are released from Tantangara Dam. This is because there is insufficient allocated water available to maintain a variable flow regime (including multiple large releases) for every day of the year.  During this time, Snowy Hydro is required to make releases to maintain flows at or above 32 ML/d at Mittagang Crossing if tributary inflows do not provide this level of flow.

Planned flows may be modified or cancelled subject to climatic environmental or operational conditions near to the time of release. The department will endeavour to inform the public of changes to high-flow releases, where possible. Whilst the volumes of smaller releases are also planned at the start of the year, the volumes of smaller releases (less than 1,400 ML/d) may be changed at any time without notice. This allows greater flexibility in the delivery of environmental releases, which provides improves environmental outcomes.

Natural inflows from tributaries will generally have a significantly larger influence on river flow rates, water levels and ecosystem processes than the environmental flows, particularly further downstream. The influence will be more significant in wetter years with more natural runoff.

Rising water levels may result from a combination of the environmental releases and natural tributary inflows. The combination of varying release volumes and natural conditions means that the peak flows in the river will vary from year to year.  

The peak will naturally reduce as it progresses downstream, but the pulse could still be detectable near the ACT border at Lobbs Hole and below.

The rise in water levels will vary depending on your location and inflows from tributaries, but flows are intended to remain within the river channel. In confined gorge reaches, the water level increase could be greater than elsewhere.

Unregulated licence holders in the Murrumbidgee I and II water sources cannot take water if flows at their nominated gauge are below the temporarily raised access thresholds published in the order for that day. Details on the raised access thresholds and how to comply can be found on WaterInsights page:

• for the Murrumbidgee I
• for the Murrumbidgee II

The commence to pump guide on those pages also explains that if flows are above the raised access thresholds, these unregulated licence holders can take water.

Stock and domestic licence holders and water users with basic landholder rights can continue to take water during the environmental flow release.

The long-term aim of upper Murrumbidgee environmental flows is to restore the section of river below Tantangara Dam to a smaller yet ecologically healthy system. The water releases help mobilise fine sediment and inundation of lower lying connected ponds to provide habitat for water dependent species. The intent is to reshape a smaller channel within the former Murrumbidgee riverbed to improve instream habitat for aquatic biota.

The flow pattern is designed to better mimic the natural flow characteristics that are typically seen in Snowy montane rivers. This flow pattern incorporates a high degree of natural seasonality and daily variability, while still maintaining natural high-flow events in the Murrumbidgee. High-flow events allow the river to re-establish stream function and improve the in-stream habitat.

High-flow events wet the riparian zone, promoting the establishment of aquatic and riparian vegetation and providing important habitat for water dependent animals, including native fish, waterbugs, frogs and platypus.

Increasing the maximum release from 1,500 ML/d to 1,700 ML/d increases the scale of benefits of moving sediment off cobbles and bettering connectivity for fish passage. This improves the habitat for the Macquarie perch spawning.

The environmental water release strategy attempts to mimic the natural hydrological characteristics of the upper Murrumbidgee River. Before the Snowy Scheme was constructed, high-flow events during winter and spring were commonplace.

Historically, the flow regime of the upper Murrumbidgee River at Tantangara showed a greater frequency of flood peaks during winter and spring.

The smaller winter flow peaks were typically associated with the passage of cold fronts delivering rain to lower elevation catchments and snow to higher elevations.

A series of smaller flow releases has been demonstrated to improve river health by establishing a new smaller channel, wetting the riparian zone and promoting the establishment of aquatic and riparian vegetation.

There is a high degree of daily flow variability with complex multi-peak hydrographs that are typical of rivers in the Snowy Mountain.

The NSW, Victorian and Commonwealth governments agreed the amount of water allocated to the entitlement each year is subject to the amount of rainfall, snowmelt and inflows into the southern Murray-Darling Basin storage dams.

Wetter conditions and stronger inflows across the southern Murray-Darling Basin may generate larger volumes of water for environmental flows. Drier years result in lower allocations and therefore less water for the environment.

Members of the public are reminded that they should exercise their own judgment about the safety of any viewing site based on the conditions on the day.

Recreational use of the river during the period in which the high flows are taking place is not recommended. These high-flow events are considered unsafe for recreational water activities as the flows may create strong and dangerous currents and could dislodge debris in the river.