A major water main ruptured in Georgetown last night and flooded portions of lower Canal Road NW, blocking traffic, trapping three motorists and threatening service to many homes, authorities said.

The actual break was near 35th and M streets, but for reasons not immediately clear, water began gushing to the surface near Canal and Foxhall Roads NW, about a half-mile to the west.

It was not certain late last night whether the 30-inch diameter underground main could be repaired without turning off water in at least some of the area it serves.

"We are going to see . . . if we can reroute water," said Libby Lawson, a spokeswoman for the D.C. Water and Sewer Authority.

If not, she said, "all of the water will be cut off in that area and we will have hundreds of customers impacted."

Just before midnight, she said, water customers were "already starting to feel the impact" of the rupture. Some reported very low water pressure and a few were not getting any.

It was not clear how long Canal Road -- a major weekday commuter artery -- would remain closed or whether its pavement was damaged.

Firefighters rescued motorists from three cars that were engulfed on Canal Road, a fire department spokesman said.

It was not known what caused the rupture, which was detected about 8:30 p.m., or how much water was lost, Lawson said. The main was one of the larger ones in a system that embraces 1,300 miles of pipes and mains ranging in diameter from four inches to more than six feet.

A water main rupture floods Canal Road NW between Key Bridge and Foxhall Road. Firefighters rescued motorists from three cars.