“All we can do is put up scaffolding to make a pontoon, and hope that the water doesn’t come up much higher,” Diane Bourlier, a 63-year-old who lives on a houseboat in Paris, said as she looked anxiously out at the rising river.

Ms. Bourlier was among the lucky ones: As of Thursday night, 400 people had been evacuated from homes in the Paris region, and a thousand faced power cuts, according to the police prefecture. Rivers have swelled across the country, forcing evacuations and the closing of roads and infrastructure.

In Paris, where the Seine rose above 18 feet on Friday, river traffic has been interrupted and roads along the river banks remained closed. A central portion of the RER C train line has been shut until the end of the month, and the government activated a plan that could let it relocate work from some ministries if the situation worsens.