Snowy River high flows 2021—frequently asked questions

The NSW Government has been working with the Victorian and Australian governments, Snowy Hydro Limited and the community to implement a program of environmental water releases to improve the health of the Snowy River below Jindabyne Dam.

How much water will be delivered?

A total of 209,577 megalitres of environmental water will be released to the Snowy River below Jindabyne Dam during the 2021–22 water year. During winter and spring, the water will be released under nine separate high-flow events. The daily flow for each of these events will be above 2,000 megalitres per day between 21 June and 9 November 2021. This will include five eight-hour peaks above 4,000 megalitres per day.

Why have nine high flow events?

The long term aim of Snowy environmental flows is to restore the river below Jindabyne Dam by scouring a smaller channel with a rocky or gravel base, better supplying pools and riffles (water running quickly over gravel beds) with fresh oxygenated water and reconnecting the river.

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

Results from previous studies from the Snowy Flow Response Monitoring and Modelling Program show that most of the physical work (sediment movement and channel maintenance) in the river channel occurred within hours of the peak flow rate. This means multiple high-flow events of shorter duration work better than one longer event.

The continuation of the pattern of multiple high flows with short duration draws on the results of previous releases which demonstrated that regular flow disturbances flush the sediment from the stream bed establishes a new, smaller channel within the former channel of the Snowy River.

Additionally, the high-flow events wet the riparian zone, promoting the establishment of aquatic and riparian vegetation and providing important habitat for water dependant animals including native fish, waterbugs, frogs and Platypus.

Why have high-flow events in winter and spring?

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

Historically, the flow regime of the Snowy River at Jindabyne 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.

The typically larger spring flow peaks resulted from rain-bearing weather systems in association with rapid melting of snow.

This release strategy provides for an increase in flow variability to better reflect the natural hydrology, with additional winter and spring high flows to represent winter and spring rainfall and snowmelt events.

Where will the water flow?

The water will be released into the Snowy River from Lake Jindabyne and travel down the river to the estuary at Marlo, Victoria. The intent is to rework a smaller channel within the former Snowy River bed to improve instream habitat for aquatic biota. The largest event has a peak of 10,362 megalitres per day which would be just in the minor flood alert zone at McKillops Bridge.

How do the 2021 high-flow releases differ from the 2020 releases?

The rainfall, snowfall and hence the stream flow of the Snowy River varies from year to year. This variability in the timing, peak flow rates, duration and volume of events is a key characteristic of Australian rivers.

The daily flow targets differ from the 2020-21 water year as a different inflow sequence was used to generate the annual release strategy. There is also more water available for environmental flow releases this year as the drought impacted water availability last year. This year there will nine high-flow events (as opposed to four events last year due to the drought), with four winter events and five in spring.

Why is there more water available than last year?

The partner governments agreed to recover from western irrigation areas a volume of environmental water for the Snowy River equivalent to 21% (212,000 megalitres) of the mean annual natural flows below Jindabyne Dam. The amount of water available each year is subject to the amount of rainfall, snowmelt and inflows into the southern Murray-Darling Basin storage dams.

The prolonged drought conditions resulted in the 2020 environmental water allocations being some of the lowest since 2011. With wetter conditions and stronger inflows across the southern Murray-Darling Basin this year, the environmental water allocation is significantly higher than the 91,476 megalitres that was available last year.

When will the releases take place and how big will they be?

Table 1. Release schedule
Monday 21 June 2021 Daily peak flow of 2,431 megalitres per day (ML/d), with equivalent peak flow 4,107 ML/d for eight hours
Tuesday 6 July 2021 Daily peak flow of 3,900 ML/d, with equivalent peak flow 5,000 ML/d for eight hours
Monday 23 August 2021 Daily peak flow of 2,080 ML/d over 24 hours
Thursday 26 August 2021 Daily peak flow of 5,000 ML/d over 24 hours
Thursday 23 September 2021Daily peak flow of 2,742 ML/d over 24 hours
Thursday 30 September 2021Daily peak flow of 2,218 ML/d over 24 hours
Wednesday 6 October 2021Daily peak flow of 5,516 ML/d, with equivalent eight-hour peak flow 10,362 ML/d for eight hours (largest flow*)#
Monday 18 October 2021Daily peak flow of 3,871 ML/d, with equivalent eight-hour peak flow 4,699 ML/d for eight hours#
Tuesday 9 November 2021Daily peak flow of 2,831 ML/d, with equivalent eight-hour peak flow 4,263 ML/d for eight hours

Note: The peak flow releases will occur during daytime hours from 8am to 4pm Australian Eastern Standard Time. The release date for these flow peaks could change as they are weather-dependent.

*The largest peak flow on 6 October 2021 may need to be moved back to later in October, subject to having enough water in Jindabyne Dam to provide adequate flow releases.

# The releases to the Snowy River will be above 2,000 megalitres per day for 13 days across the nine high-flow events. For the two October high-flow events there will be days leading up to and/or following that will have flows larger than 2,000 megalitres per day to maximise the ecological benefits for the river.

How high will the water rise in Snowy River?

The peak flow rate for the largest high-flow event on 6 October 2021 of 10,362 megalitres per day is roughly twice the size of the 2020 peak event. The peak will naturally reduce as it progresses downstream, but could still be detected near the mouth of the Snowy River at Jarrahmond in Victoria.

The maximum annual flow volume is dependent on both the weather and annual water determinations in the southern connected Murray–Darling Basin, therefore peak flow releases will vary from year to year. In some years they may be higher or lower than the peak flow rates used to date.

We anticipate that during the largest of the spring 2021 events on 6 October, the river level will rise from 1.10 metres to approximately 2.01 metres at the Dalgety gauge. 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 slightly greater than elsewhere.

The NSW Government recommends that equipment should be secured or elevated three metres higher than the existing base water level in the Snowy River.

Can I view the flows?

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. The township of Dalgety and the Snowy River at Jacks Lookout have suitable viewing locations.

Can I swim and use my water sports equipment anywhere along the river?

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 could dislodge debris in the river.

What actions should I take?

Individual landholders should take all necessary precautions with stock and property, both before and during the additional flows. This may involve moving any assets away from the bank of the river.

What is the NSW Government doing to ensure the flows are delivered safely?

Landholders are advised to take all necessary precautions with their stock and property, both before and during the flows. The NSW Department of Planning, Industry and Environment, along with Snowy Hydro Limited, will continue to monitor flow conditions—both the storage releases and any natural downstream inflows occurring at this time.

The Snowy River high flows 2021—frequently asked questions PDF, 210.17 KB fact sheet is also available to download.