A 16-inch water main broke beneath Georgia Avenue in Wheaton yesterday, flooding basements, cutting water pressure to scores of homes, shutting down part of the avenue and backing up evening rush-hour traffic.

Authorities said they expected to repair the break by 3 or 4 a.m. today.

They said it was necessary to cut off water to a "very few" homes in the immediate area of the rupture while the work was in progress.

The leak occurred about 4:30 p.m. at Georgia Avenue and Lindell Street and forced the closing of the southbound lanes of the avenue in that area.

A Washington Suburban Sanitary Commission spokeswoman said that work crews tore up a "pretty big share" of the roadway during last night's repairs.

Repaving was expected to begin some time after 6:30 a.m., and the reopening of the street was expected later today, the spokeswoman said.

For a time it was believed that the main had ruptured in a second spot, about a quarter mile north of the first break.

However, it was later found that the apparent second leak involved only water from the initial break, which had backed up.

The WSSC spokeswoman, Marjorie Johnson, said she did not immediately know the cause of the leak. Meanwhile, neighborhood residents noted a variety of effects.

"I'm not letting anybody have a bath," said Catherine Stier of 2402 Sheraton St. "The water's too rusty looking."

"I've got water in my basement galore," said Geraldine Venable, who lives a few blocks to the south, on Lindell Street.

"I'm exhausted," she said, describing her attempts to cope with the flooding.

"I've been in this house 34 years and never had anything like this.

"It's just been a nightmare."

Another Lindell Street resident said cars being diverted from Georgia Avenue were filling her street.

"It looks like a parking lot," said Phyllis C. Mertz. "Traffic is incredible."

The WSSC spokeswoman described the broken 16-inch main as a "rather large" conduit, capable of supplying hundreds of homes and businesses.

She said valves make it possible to isolate breaks and reroute service, and water was shut off last night only to premises in the immediate area of the intersection where the rupture occurred.

Most of the supply system consists of prestressed reinforced concrete pipe, she said, but yesterday's break involved cast iron pipe.

She said that the break involved a split, or horizontal crack in the water pipe.

Witnesses said that the break sent water spurting up through cracks in the ground to a height of about eight inches. Spectators gathered. Passing automobiles sent plumes of spray up to the cars' rooftops.

Children blowing whistles rode bicycles through the streaming brown water.

Firefighters arrived in response to calls about flooded basements. Police directed traffic in the glow of flares.

One resident said there had been several water main breaks in the area in the last year.