Riverside homes and businesses in Paris are on high alert as the swollen River Seine threatens to overflow its banks.
Weeks of rainfall have produced a relentless rise in the water level, which is expected to peak just below 6m (20ft) above normal.
Touring boats are tied up, riverside roads are sealed off and the Louvre museum has closed a lower gallery.
France has seen rain like this over the New Year period only three times in the last century.
The surging brown waters are also reportedly flushing rats out of their usual haunts below ground, the BBC's Kevin Connolly reports from the French capital.
Within Paris, the Seine runs in a deep channel which limits the effects of the rising waters. But in smaller towns along the river, our correspondent adds, shoppers and commuters have been punting boats along flooded streets. They are waiting for the waters to recede to allow the first estimates of the financial cost of the flooding to be made.
A statue of a French soldier from the Crimean War- known as The Zouave - on the Pont de l'Alma has long been used as a marker for water levels in the city.
On Saturday afternoon, the water was still well below his waist; during the historically bad floods of 1910, when the city was submerged for two months, it reached his neck.
As of 09:00 (08:00 GMT) on Saturday, the river level had reached 5.7m above normal.
While forecasters believe it will continue to rise, peaking on Sunday night or Monday, it is not expected to reach the 2016 high of 6.1m, AFP news agency reports.
Saying the city was coping, Mayor Anne Hidalgo suggested the flooding, coupled with recent summer heat waves, was "clearly a question of the town adapting to climate change".
She warned that the high water levels would remain into next week, as water levels subside slowly due to waterlogged soil in the region.
At the Louvre, a lower level housing Islamic artwork was closed to visitors. Other famous attractions like the Musée d'Orsay and the Orangerie gallery were on high alert.
In the prestigious 16th Arrondissement (district), some basements in residential buildings were slowly being inundated.
"There are six studios in the basement, and we've had to set up blocks outside to keep the windows from breaking and covering everything in water," one caretaker, Joao de Macedo, told AFP.
Inside the studios, tables and dressers have been lifted off the floor as water seeps through the walls, the agency says.
Outside, a young woman said it was "great to see ducks instead of cars" in places.
A health centre in the north-western suburbs, where 86 patients were receiving care, was evacuated on Friday.