A water main near the Rosslyn Metro station burst early yesterday morning and flooded the station, disrupting service on portions of the Orange and Blue lines for several hours, forcing thousands of commuters to find other ways to get to work and snarling rush-hour traffic.

Metro spokeswoman Marilyn Dicus said that after the 3 a.m. break, water from the underground pipe at Fort Myer Drive and Wilson Boulevard spilled down Wilson into grates above the station. Transit officials were forced to shut down rail service between Ballston and Foggy Bottom and between the Pentagon and Rosslyn stations.

Dicus said Metro provided 20 extra buses for stranded commuters and made announcements over Metro public address systems advising passengers of alternate routes.

The Orange Line reopened at 8:11 a.m., Dicus said, and Blue Line service resumed at 9:28 a.m. At one point, water in the control room of the Rosslyn station was 4 feet deep, officials said.

The flooding caused "some water damage, but nothing major," said Metro spokeswoman Beverly R. Silverberg. "The water was mostly in our automatic train control room. A lot of equipment got waterlogged, and we had to check it all out," delaying the resumption of Orange and Blue line service, she said.

Communications and electronic signal equipment used to operate the trains was knocked out along the line by the flooding.

H.T. Angell, Water and Sewer Division chief for Arlington County, said he did not know what caused the cast iron pipe with a diameter of 12 inches to split. The 10-year-old water main is considered relatively new in the county's 60-year-old water and sewer system, he said.

Typical breaks are triggered by changing temperature and usually involve leaking joints, Angell said, but in this case, "a piece of pipe split lengthwise. The water is under a great amount of pressure. Once it splits, the break gets bigger in a hurry."

For many commuters on the busy Orange and Blue line corridors, the break meant a frantic hunt for rides and a prolonged rush hour on roads that, even on good days, are plugged with morning traffic into the District.

Cristina Rosenberg of Arlington said her usual half-hour commute to work in Bethesda stretched to an hour and a half yesterday.

By the time she arrived at the Orange Line's Court House station, "all the buses coming in from Ballston were full," Rosenberg said. "A lot of people were left just stranded, trying to catch rides."

Rosenberg said she finally walked to Rosslyn and caught a bus there. "It was a disaster -- it's a good thing it was not cold," she said.

"I was there," sighed Cpl. John Haas of the Arlington police, who stood with three other officers to direct traffic in the congested intersection of Wilson Boulevard and Fort Myer Drive from 7:15 until 10 a.m. "It was bad."

Rush-hour snarls that usually vanish by 9 a.m. persisted for an extra hour, Haas said. "It was real slow," especially on Lee Highway and eastbound Wilson Boulevard, he noted, as water repair crews temporarily blocked two of three lanes and Metro shuttle buses added to the snaking traffic.

The break did not disrupt water service to area buildings, officials said. Water division crews spent about four hours repairing the break, a job that probably will cost the county between $500 and $1,000, Angell said.