Adelaide University's Professor Mike Young's appeal to "downsize" the river system included the possible abandonment of some of Victoria's prized river red gums in the Barmah forest.

Speaking at a water conference in Melbourne yesterday, Professor Young said a drier future would force Australians to choose which environmental assets to keep, and which to let die.

"At the moment we are pretending we can keep it all, with half the amount of water, but I can't see how we are going to do that," he said.

"If we keep on trying to spread half the water over the same area, we are going to lose everything. It's time to think about reconfiguring."

Professor Young identified Victoria's Lake Mokoan and New South Wales' Menindee Lakes as examples of subsidiaries to the river that only added to its size and evaporation problem.

He said tough decisions would also be needed at the Barmah State Forest where Victoria's ancient river red gums rely on increasingly rare floods for survival.

"Would we be prepared to put bunds across parts of the Barmah forest and decide if it remains dry we are only going to keep one-third of it alive? Rather than spreading water like Vegemite over the whole system, we could pick areas to keep healthy so we don't lose it all," he said.

His comments follow a decade of perilously low rainfall, emergency measures being conducted at the lower lakes, and revelations that February had the lowest inflows to the system in the 117 years since records began.

Professor Young found support yesterday from a key red gum adviser to the Victorian Government: Johan Van Rensburg from engineering firm GHD.

Mr Van Rensburg has conducted modelling for the Brumby Government's red gum strategies, and said Professor Young had "raised a good point".