A portion of Route 29, which runs along the river on the New Jersey side and of River Road in Pennsylvania were closed, WCAU-TV in Philadelphia reported.
Observers said the ice, large chunks crowded together, covered the entire river for at least a mile. The National Weather Service issued a flash flood warning for the 30-mile stretch of river between Trenton and Philadelphia.
"When this ice jam breaks, there may be enough water behind the jam to cause flash flooding downstream," the NWS said.
Temperatures in the area got down into the single digits Tuesday and never got above 12 degrees. Wednesday was slightly warmer but still well below freezing.
The jam might break up Thursday afternoon as the mercury rises into the mid-30s, forecasters said.
A similar ice jam formed in the Lehigh River, one of the Delaware's major tributaries, the (Allentown) Morning Call reported. Part of Lehigh Drive in Easton, Pa., was closed, and a woman had to be rescued from her car after it got stuck in water on the roadway.