RAISING the Wyangala Dam wall by 10 metres will add 650 gigalitres of storage capacity to the reservoir but historical data suggests that capacity will rarely be used.
The dam has filled only four times since 2000 and according to government modelling the raised wall would yield just 21GL annually.
Construction of the dam began in 1928.
It was completed in 1935 and then extended in 1971, tripling its capacity to about 1200 gigalitres.
For the proposed extension there is no concept design, business case or feasibility study. The expansion is a 650GL capacity, but will yield only 21.02GL on an annual long-term average. That is, it will fill only about once every 25 years.
That the extra room in the dam will rarely be used angers Upper Lachlan farmers who will likely lose coveted river flats from their business models.
But Lower Lachlan water users have much greater concerns for the system upon which they rely for their stock and irrigation water, and about greater environmental affects raising the wall will have.
Gordon Turner, whose family properties north west of Hay include sections of the Booligal wetlands and the major Lower Lachlan wetlands complex below Booligal, says waterbird habitat will suffer greatly.
The Booligal wetlands are extensive, covering about 15,000 hectares.
They are nationally important and are recognised for the large waterbird colonies they sustain.
They form a part of the extensive lower Lachlan river wetlands and rely on river flows from the upper Lachlan, and the Belubula and Boorowa rivers.
Mr Turner says the wetlands rely on sporadic major flood events for their survival.
And the relationship between flooding on the Lachlan below Wyangala and the shallow acquifers that provide groundwater are not well understood.
"That's one of the questions we're asking: do the acquifers recharge in major flood events?"
Mr Turner says logic would suggest sporadic long-term flood events between Cowra and Oxley would seep into acquifers and government studies state the same.
"We just want a proper job done of all this. The government seems hell bent on building infrastructure, that we have no problem with, it just needs to serve the purpose for which it was built," said Mr Turner.
He said it seemed the National Party was driving the bid to raise the Wyangala Dam wall.
"Sometimes it seems they're the political wing of the NSW Irrigators' Council," he said.
Mr Turner said environmental flow provisions of the Lachlan Water Sharing Plan combined with state and Commonwealth waterholders' volumes could not replicate a flood required for a major bird breeding event in the lower Lachlan.
"Raising the dam wall could be Australia's largest-ever man-made environmental disaster."
Mr Turner said the current water sharing plan for the lower Lachlan struck a good balance between environment and extraction, but the Wyangala Dam expansion would mean major changes to that plan.
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