Eight people were rescued from their vehicles as water poured into the expressway just before 3:30 a.m. Thursday.
The main broke on Jerome Avenue near East 175th Street.
Water flooded on the Cross Bronx Expressway below, closing the roadway in both directions.
Occupants of at least seven cars were trapped in their vehicles.
Witnesses said the water was so high some of the cars were actually floating.
Other cars were stuck in rising waters on Jerome Avenue.
There have been no reports of injuries.
Jerome Avenue is closed from Mount Hope to the Cross Bronx Expressway.
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