"As a result of the persistent cold, river ice will be thickening and increasing," threatening flooding in neighborhoods along streams blocked by ice jams, the service concluded.
"It's the first time in my three-year tour here that we've seen so much ice," said Lt. Veronica Smith, chief of waterways management for the Coast Guard's Philadelphia station.
The Coast Guard says a little ice forms along the riverbanks most years in early January and is usually gone by February, while "severe disruptions" threaten to block shipping every seven to 10 years.
To keep ships and barges moving on the Delaware, the Coast Guard has stepped up icebreaking operations by its 65-foot-tugs Cleat and Capstan, and the buoy tender William Tate, said Smith.
If the cold continues and ice keeps piling up in the Delaware, covering the open water, "the next steps would be our plans for convoy operations, where our icebreakers would lead vessels into port," Smith said. "We're monitoring it."
The Coast Guard says ice is fast to the shore around the bases of the Walt Whitman and Ben Franklin Bridges over the Delaware; "pack" ice has piled two to four inches deep on most of the surface of the Schuylkill under the Girard Point Bridge; "drift" ice up to an inch thick covers nearly half the Delaware near the Marcus Hook oil terminals, with smaller ice volumes reported near Chester and below Wilmington, where the river widens into the Delaware Bay.
The Coast Guard started breaking ice three miles below Trenton, when that area was threatened by frigid water rising behind a dam of piled ice during a cold snap Jan. 9.
On Thursday, the Coast Guard banned all but steel-hulled vessels from the Delaware north of the Tacony-Palmyra Bridge and from the Schuylkill, the Salem River in South Jersey, and the Chesapeake and Delaware Canal, which cuts through Delaware and Maryland south of Wilmington.
River traffic also has been restricted on the frigid Schuylkill by recovery operations at the site of last week's CSX train derailment at the 34th Street bridge near the Philadelphia Energy Corp. refinery, the Coast Guard said.