The Delaware River is backing up behind an ice jam near Trenton, sending water overflowing it banks on both sides of the river.
The National Weather Service said flooding has caused multiple road closures along Route 29 in Trenton.
Flooding was also reported on the Bucks County side of the Delaware River between Morrisville and Yardley with portions of River Road closed near the Calhoun Street Bridge, Richard Road, Black Rock Road and Ferry Road.
The NWS says flooding could occur in Ewing and White Horse in New Jersey, and Woodside in Bucks County.
A flood warning is in effect until 5:30 p.m. north of Trenton.
In issuing the flood warning, the NWS said the jam was "causing a significant rise in the level of the Delaware River at Trenton."
A flood warning means flooding is imminent or occurring.
Trenton residents in the meantime were advised by the state environmental officials to boil their water "due to inadequate treatment."The Trentonian said the city should have issued the advisory, including making robocalls, but did not.
The forecast for Monday calls for cloudy skies with a high of about 30, neither of which will assist in melting any ice. There's a chance of snow followed by rain Tuesday.