Flood peak passes Bourke on way to Menindee Lakes

A minor flood peak of near 11 metres passed through Bourke this week, causing some minor inconvenience, and giving the river a good flush.

At the time of writing (Tuesday 27 April), the Darling River at Bourke was 10.93 metres and rising slowly.

The volume of water passing Bourke on Tuesday was just over 43,000 megalitres per day – or 43 billion litres per day.

According to WaterNSW, over half a million megalitres, or 500 billion litres, has already flowed past Bourke during this flood. This water, on its way downstream to Louth, Tilpa, Wilcannia, and the Menindee Lakes is equal to the capacity of Sydney Harbour and there’s much more to come.

The Darling River at Louth was 9.04 metres and rising on Tuesday, above the minor flood level, and is expected to peak at 10.60 metres in early May, with moderate flooding.

At Tilpa, the Darling rose above minor flood level (9 metres) on Monday and is expected to reach 11 metres in early May, with moderate flooding.

The Darling River at Wilcannia was 7.79 metres on Tuesday, and the river there is expected to reach around 9.40 metres in late May with minor flooding.

After a good flow in the Darling River last year, the Menindee Lakes had received over 650,000 megalitres of water – about a third of total capacity of the lakes.

But due to water use in the Lower Darling River and the massive summer evaporation rates of the Lakes, this volume had dropped to 300,000 megalitres by March 2021.

WaterNSW is currently expecting up to 900,000 megalitres to flow into Menindee Lakes bringing the total in the Lakes to well over 1 million megalitres.

Menindee Lakes storage comprises four main lakes – Cawndilla, Menindee, Pamamaroo and Wetherell – and several smaller lakes with a combined capacity of 1,731,000 megalitres, three and half times the capacity of Sydney Harbour.

The Menindee Lakes can lose up to 700,000 megalitres per year to evaporation when they are full. Average annual evaporation losses are over 400,000 megalitres per year.

Spokesman for Barwon-Darling Water Ian Cole said that evaporation losses from Menindee Lakes had long been a concern for successive state and federal governments.

“The lakes are in a semi-arid area and are extremely shallow storages with huge surface areas. In most years they lose the equivalent of one Sydney Harbour of water to evaporation,” Mr Cole said.

“This huge loss of such a precious resource is taken into account when governments are making decisions about releasing water from the lakes,” he said.