The nursery rhyme about London Bridge falling down might have been applied yesterday to the Woodrow Wilson Bridge. It developed a monster pothole that snarled morning rush hour traffic for nearly four hours and, according to the American Automobile Association, caused damage to 20 cars.

The hole that resulted when paving crumbled in the center of the bridge's three Maryland-bound lanes measured seven by eight feet, according to James Bryce, project engineer for Cianbro Corp., the company that is repairing the decrepit 22-year-old bridge.

AAA spokesman Tom Crosby said he surveyed the damage and could see through to the Potomac River. He said the only things that kept small cars from dropping through to the water were the remaining steel rods that had reinforced the concrete. Crosby said two cars had to be towed away after hitting the hole, and 18 more sustained flat tires and dented bumpers.

Bryce described the hole as the largest that has developed on the bridge and said it began Monday as a series of small potholes that were patched twice during the day. He said that on this part of the span, near the Maryland shore, the constant pounding of traffic, combined with the recent snow and rain, perhaps aggravated by de-icing chemicals, are causing the pavement to deteriorate rapidly.

"There's no strength in the concrete . . . Areas just drop out of the bridge," he said.

Maryland State Police reported the hole to Cianbro Corp. about 6:30 a.m., and within two hours workers had placed two 10 foot-by-four-foot steel plates over the hole, Bryce said. Traffic remained snarled until almost 10 a.m., according to state police.

Gary Oertly, night superintendent for Cianbro, said late last night that the patching of the pothole would be completed early this morning and that it should pose no problem for today's rush hour.

Cianbro, which began repaving the bridge with prefabricated concrete slabs last December, has completed about a fourth of the job and expects to have it finished by September, about three months ahead of schedule, Bryce said.

The Wilson Bridge carries the Capital Beltway across the Potomac between Alexandria and Prince George's County.

The American Legion Bridge, also called the Cabin John Bridge, which carries the Beltway across the river between Montgomery and Fairfax counties, is also plagued by potholes and is scheduled for repair next year.