A year in the Snowy and Snowy montane catchments: 2022–23

Under a wet resource availability scenario, environmental water managers and the Snowy Advisory Committee for the Snowy and Snowy montane catchments developed the Annual plan for the Snowy and montane rivers increased flows 2022–23.

Objectives for the use of water for the environment were to:

• provide daily variable flows for the Snowy and upper Murrumbidgee rivers with higher release rates during winter and spring that are typical of a snowmelt river
• improve the Snowy River channel condition by providing several high flow events to reduce fine sediment and algae smothering, and clean riffles, cobbles and gravel by mobilising sediments
• provide 3 high flow releases in the upper Murrumbidgee River to flush gravels and cobbles and create better habitat conditions before the Macquarie perch (Macquaria australasica) breeding season
• inundate and connect low-level ponds and slack water areas during late winter and spring, and provide freshes in summer to support breeding and recruitment opportunities for flow-dependent frog species and other native fauna
• provide natural daily flows to targeted Snowy montane streams to provide connectivity and maintain ecological functions.

With very wet conditions and high environmental water allocations, environmental water managers provided opportunities for water-dependent native plants and animals to breed and recruit in the Snowy and Snowy montane catchments.

These tables provide a summary of water for the environment delivered in the Snowy and Snowy montane catchments during the 2022–23 watering year.

Snowy River below Jindabyne flows

Snowy Montane Rivers Increased Flows

Catchment total

<sub>Notes: 
* The allocation of Snowy River Increased Flows was 215,310 ML. Of the allocation, a maximum of 212,000 ML is recognised by Snowy Hydro Limited for delivery. The Department of Climate Change, Energy, the Environment and Water is working on the status of the remaining balance of 3,310 ML. 18,700 ML was deducted from the 212,000 ML to repay a spill borrow from the 2021–22 water year.</sub>

_{** Very high inflows into Jindabyne Dam resulted in the dam spilling between 28 October 2022 and 2 November 2022, and 5 and 11 November 2022, with several periods of pre-releases (controlled spills for flood mitigation) from October 2022 to January 2023. Of the total 270,506 ML of water spilt from the dam, an estimated 16,000 ML will be treated as Snowy River Increased Flows ‘borrow’ to be repaid from future allocations.}

_{Tributary inflows also resulted in multiple flooding events in the lower Snowy River.
The Snowy Montane Rivers Increased Flows (SMRIF) allocation to the Snowy montane rivers is scaled by a pre-determined gigawatt hour electricity opportunity cost, which is different for each of the river reaches. Except for the upper Murrumbidgee River, the volume of SMRIF debited is an agreed annual average. Each year the actual flows that pass through the delivery points change subject to the conditions. For example, the volume will be lower in dry years and in wet years, such as 2022–23, the flows will be higher. The upper Murrumbidgee releases are adjusted as required from Tantangara Dam.}

In 2022–23 environmental water managers worked with stakeholders and the community to deliver water for the environment to the Snowy, upper Murrumbidgee, Geehi and Goodradigbee rivers. Environmental flows supported river connectivity to improve river health and provided important habitat for water-dependent native plants and animals including frogs, platypus and fish.

The high flows improved the health of the Snowy River by cleaning riffles and flushing sediments.

The high flows played a crucial role connecting ponds and small wetlands along the Snowy and upper Murrumbidgee rivers. Connection of these habitats supported breeding activities for a diversity of frog species. Water managers conducted frog monitoring and found 7 frog taxa, including the threatened alpine tree frog, in the Snowy and Snowy montane rivers.

The alpine tree frog, a subspecies of the broadly distributed whistling tree frog, is listed as endangered in New South Wales and vulnerable under Commonwealth legislation. This species occurs in alpine and sub-alpine zones or high country in south-eastern New South Wales and Victoria, usually above 1,100 m and mostly located within Kosciuszko National Park. The species was found at multiple locations along the upper Murrumbidgee and connected ponds. Monitoring teams also observed recently metamorphed juvenile frogs in December 2022.

Snowy River flushing flows

The development of the Snowy Hydro Scheme significantly altered the natural flow regime for the Snowy River. From 1967 to 2002 around 99% of natural flows in the upper Snowy were diverted for power generation and irrigation purposes in the Murrumbidgee and Murray catchments.

Environmental releases first started in the Snowy River in 2002. Environmental water managers designed high and flushing flows to be delivered to encourage scouring and in-channel erosion, with the aim of redefining the heavily sedimented river channel.

The long-term aim of this process is to return the Snowy River to a smaller, healthier montane river.

The physical riverbed structure, or geomorphology of the Snowy River channel has been monitored on several occasions since environmental flows commenced in 2002. This monitoring helps water managers understand how the river channel changes over time and whether environmental releases are helping the Snowy River become a healthy system again.

Environmental water managers work with other agencies including the Department of Climate Change, Energy, the Environment and Water, the Bureau of Meteorology, State Emergency Services and Snowy Hydro Limited to reduce or postpone releases to avoid water levels that are higher than desired. In 2022–23, several events were superseded by significantly larger flood mitigation releases and one event was significantly reduced in size due to predicted downstream tributary inflows.