Sports minister Amélie Oudéa-Castéra praised Paris' ability "to provide athletes from all over the world with an exceptional setting on the Seine for their events."

Last year, swimming test events had to be cancelled due to poor water quality. One reason was heavy rains that overwhelmed the city's old sewers, causing a mix of rainwater and untreated sewage to flow into the Seine and leaving safety standards unmet.

The giant reservoir dug next to Paris' Austerlitz train station aims to collect excess rainwater and prevent bacteria-laden wastewater from entering the Seine.

It can hold the equivalent of 20 Olympic swimming pools of dirty water that will now be treated rather than being spat raw through storm drains into the river.

“We are on time," the prefect of the Paris region, Marc Guillaume, said on Thursday.

“The beginning of the Games will coincide with water quality allowing competition. That's a tremendous collective success.”

Paris mayor Anne Hildago promised she would herself swim in the Seine before the Olympics — possibly alongside President Emmanuel Macron.

The new storage basin “guarantees” that water can be stored even during severe storms, and will help water levels to "return to normal as quickly as possible,” she said.

The opening of the basin is the latest step toward a cleaner river and comes as part of a series of newly-built facilities, including a water treatment plant in Champigny-sur-Marne, east of Paris, that was inaugurated last month.

During the Olympics, water will be tested at 3 a.m. each day to determine whether events can go ahead as planned. If results were not up to the standards, events could be delayed by a few days, organizers said.

The estimated cost of the cleanup efforts amount to 1.4 billion euros ($1.5 billion), paid by the state and local authorities.

“For more than ten years already, we've seen a very significant improvement of the Seine water quality and our river's fishes and wildlife are back,” Hidalgo said.

About 35 fish species are now living in the Paris section of the river, up from only three in the 1970s, when waters were extremely polluted due to nearby industrial activities.

For decades, the Seine was used mainly as a waterway to transport goods and people or as a watery grave for discarded bicycles and other trash. Swimming there has, with some exceptions, been illegal since 1923.

Paris officials are planning to open several bathing sites to the general public in the summer, starting from next year.

The River Seine also is to be at the heart of the grandiose opening ceremony for the Olympics that will see over 200 athletes' delegations parade on more that 80 boats in central Paris.