Hurricane Isaac moved along the south east coast of Louisiana on Wednesday, battering New Orleans with heavy rains.
Winds hit speeds of up to 120/km an hour.
Authorities in the US state say nearly half a million homes and businesses are without power.
While the flooding is widespread, the Category 1 hurricane has so far failed to breach the barriers erected to protect New Orleans after the Katrina disaster seven years ago.
But seawater has already run over one levee to the south of the city.
The storm is heading northwest towards the Mississippi-Alabama order where hurricane warnings remain in place.
Forecasters, however, expect Isaac to weaken to a tropical storm early Thursday morning.