Eljayel, you appear to be taking a literal interpretation of the term "flush". A seasonal flush is merely a term for a seasonal increase in flow. So such an increased flow through recycling water numerous times will merely duplicate what the melted snow does.

Furthermore, the water is clean at the start and is generally just as clean, if not cleaner, when it is recaptured down stream. But if one tried to do this sort of recycling with certain third world rivers that are essentially used as open sewers, then I agree, the last cycle certainly would be something to behold, as you put it.

The Pearl River in Gwongdung, the Ganges at Varanasi, the one in Bangkok and some of the less salubrious reaches of the Parramatta and Yarra come to mind.

That is not the case with the Snowy River and it is highly unlikely that there would be many rivers, or parts of rivers, in Australia that would have pollution levels that would preclude this sort of recycling. And even if there was, the regular capture of the flow would facilitate the application of both active and passive filtration systems etc to the cycle prior to reintroducing the water to the river.

Generally, the flow of water in the upper reaches of river systems will actually improve oxygen levels as it drops over small falls etc.

thanks Perseus, that seems pretty simple. But if the flush does nothing, then why do it? It seems a huge waste of resources.
Of course, if it was necessary, then filtering would solve the problem of waste build-up. I suppose it is easy to filter 3000 megalitres a day.

The reason for the flush, Eljayel, is that some of the species that live in the river have evolved to complete part of their life cycle when the river is flowing well. Most can still get by on reduced water flows but if we can find ways to meet the needs of both wildlife and farmers at the same time then we certainly should consider it.