Flooding in the Barmah–Millewa Forest depends on flows in the River Murray. A natural narrowing of the river (known as the Barmah choke) restricts flow and results in overbank flooding when flows downstream of Yarrawonga Weir exceed the channel’s capacity. This restriction influences both the operation of Yarrawonga Weir and the upper limit of environmental flows that can be delivered to the forests.
The delivery of irrigation water during summer and autumn is managed to minimise unseasonal flooding of the forest. Regulators along the River Murray remain closed during summer and autumn to restrict flow through low-lying flood runners. The delivery of water to Barmah Forest is also limited by an imposed flow constraint downstream of Yarrawonga Weir to prevent flooding of private land. The current constraint limits releases to a maximum of 18,000 ML per day until the end of September (with potentially affected landholder support) and to 15,000 ML per day for the rest of the year. To overcome this constraint, most environmental flows are directed into Barmah and Millewa forests via regulators rather than overbank flows. It is currently not possible to achieve the desired flooding depth and duration for floodplain marsh vegetation in both forests at the same time without natural flooding.
Water management at Barmah–Millewa Forest seeks to build on unregulated flows and the delivery of consumptive and operational water en route to optimise environmental outcomes when possible. As Barmah–Millewa Forest is located towards the upper reaches of the regulated portion of the River Murray, water for the environment delivered to the forest can often be used again at sites further downstream, as part of multi-site watering events.