Managing water for the environment in the Snowy and montane rivers

Managing water for the environment

Snowy River

The partner governments agreed to recover from western irrigation areas a volume of water for environmental flows for the Snowy River equivalent to 21% of the mean annual natural flows (MANF) below Jindabyne Dam prior to its diversion, or 212 gigalitres per year, with a future commitment to review the need for flows of up to 28% MANF.

Snowy environmental flows aim 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 to allow food to wash down, and animals to move along its course.

A ‘natural flow scaling’ approach has been used to deliver water for the environment to the Snowy River below the Jindabyne Dam since 2013. This approach aims to release water with variations over time, or a flow pattern, similar to that observed in the Thredbo River, which is of a similar size to the new, smaller Snowy below Jindabyne.

The interim objective using this ‘natural flow scaling’ approach is to provide flows that over the long term will drive the river channel characteristics of the Snowy River below Jindabyne Dam to be more like the Thredbo River as fine sediment is mobilised and redistributed.

Montane rivers

On the upper Snowy, Goodradigbee, and Geehi rivers, small weirs divert water from tributary creeks into Snowy Scheme storages. For selected locations, some of the diverting structures have been closed to allow water to again pass along these creeks to the main rivers. Environmental flow releases are also made from Tantangara Dam into the upper Murrumbidgee River.

The montane river releases aim to restore natural flows that connect with the main river, to improve local river health, promote productivity for aquatic food webs and encourage reproduction and movement of native animals including water bugs, frogs and native fish.

Water allocation to the Snowy montane rivers is scaled by pre-determined gigawatt hour electricity opportunity cost for the different river reaches. Each water delivery release point in the Snowy montane rivers is taken to equate to a calculated amount of foregone electricity generation for Snowy Hydro Limited.

The power generation that Snowy Hydro Limited foregoes is limited to a maximum of 150 gigawatt hours per annum. The amount of water released to the montane rivers is proportional to the water available for the Snowy River Increased Flows (SRIF) for that year.

Water is allowed to flow through some control structures, replacing natural flow patterns to keep the Snowy montane rivers healthy. Available water is released from Tantangara Dam into the upper Murrumbidgee through a controlled outlet.

Each January, SRIF annual allocations are determined by inflows into the western catchment storages where various Snowy water entitlements are held. Flow releases to the Snowy River are to deliver 212 gigalitres (GL), or 21% of the average annual natural flow. When low water allocations are available in the western storages, less water is available for the Snowy and montane rivers.

SMRIF are provided to the montane rivers including the Snowy River above Jindabyne Dam, Goodradigbee River, Murrumbidgee River below Tantangara Dam and the Geehi River. Each year, the allocation of water to these montane rivers is proportional to the volume of water allocated to the Snowy River, scaled by the electricity generation potential of the release point. If 212 gigalitres is available for the Snowy River, the full 150 GWh of generation potential equivalence will be allocated to the Snowy montane rivers, or 118 gigalitres.

Outcomes in the Snowy and montane

Jindabyne Dam was constructed in 1967. The purpose of the dam is to redirect the eastern flow of the Snowy River west for irrigation and to generate hydroelectricity via the Snowy Murray development of the hydro scheme. This resulted in Snowy River flows below the dam wall being reduced to 1% of their annual mean flow until 2002. The characteristics of the river channel changed greatly from a high-energy river in a rocky bed; silting up and becoming wide and shallow with woody plants encroaching across a sandy river-bed.

With the return of water as Snowy River Increased Flows, flushing flows are used to scour and transport sediment (silt, sand, and clay) to reclaim a deeper more rapid-flowing channel within the old Snowy river bed, and to restore river health and conditions more typical of a snow-melt mountain river. Over time, the reintroduction of these channel-forming flows is improving habitat for plants and animals that call the Snowy River home.

Flows in the Snowy montane rivers boost food productivity for native plants and animals. Releases in these high-altitude alpine rivers have improved water quality and provided connectivity promoting reproduction and dispersal of frogs, native fish and water bugs.

How the process worked in the past

The Snowy Water Initiative was formally established in 2002 to achieve significant improvements in river health by releasing environmental water into the Snowy and montane rivers. Advice on how best to deliver the flows was initially provided by the Snowy Scientific Committee. In 2014 legislation was passed to replace the former Snowy Scientific Committee with the broader representation of a Snowy Advisory Committee.  

In February 2019, the SAC provided its first formal advice for the delivery of environmental water for the 2019/20 Snowy water year, which covered the period 1 May 2019 to 30 April 2020.