The Olympic open-water races and triathlon swims will be held on a scenic stretch of the river through the heart of Paris where last summer the bacteria levels passed the city's safe maximum.

The cleanliness of the river is highly dependent on the weather. Heavy rain washes pollution off the banks and also forces drains and waste pipes to back up into the river.

Storms just before events could lead to Olympic postponements or cancellations.

As she opened a rainwater treatment on the Marne, which flows into the Seine just east of Paris, Sports Minister Amelie Oudea-Castera on Tuesday declared that ensuring the two rivers are fit for swimming was "one of the challenges of the century".

She pointed out there were 94 days to go to the Olympics.

"We are ready, on time," she said. "We will be ready to meet our legacy."

The plant is designed to return clean water to the Marne.

"The equivalent of an Olympic swimming pool per hour," said Olivier Capitanio, President of the Val-de-Marne Department.

A huge cistern for overflow water was opened at Austerlitz on the eastern fringe of central Paris last May. It can hold the equivalent of 20 Olympic swimming pools worth of water.

The measures to clean up the two rivers should also enable local residents to bathe in them, something they have not been able to do in the Marne since 1970.