THE biggest release of water ever seen from Jindabyne Dam has begun, as an extra 84 billion litres of melted snow thunders down the Snowy River over the next three weeks.

The release is designed to scrape mud, weeds and silt from the Snowy's clogged bed. It will build up to a peak on Wednesday when the newly concreted spillway on the face of the dam is fully opened for the first time since the dam was completed in 1967.

And the water came thundering down ... John Gallard, the chairman of the Snowy River Alliance, watches the water flooding over the Jindabyne Dam spillway, exploding as it hits the river bed yesterday. Credit:Andrew Sheargold

The timing of the flow is designed to mimic the spring snow melt, the NSW Office of Water said.

A warning was issued to people living downstream that the Snowy could rise between 1.8 metres and 3.6 metres, as river flows rose from 450 million litres a day to 12 billion litres at the peak of the environmental release.