A relentless downpour turned a section of the Capital Beltway into a tangle of wrecked tractor-trailers yesterday, played water polo with traffic at various locations and washed out a bridge in Montgomery County -- all without reported injuries.

It was the third day of rain out of the last four for the Washington area, with the latest downpours fed by a tropical depression that moved inland over the South Carolina coast Monday night.

On a winding, two-mile stretch of the Beltway between Connecticut and Georgia avenues, there were four tractor-trailer jackknifings -- three of them within 20 minutes. There were scores of minor traffic accidents; a smattering of power outages were blamed on tree limbs that fell on power lines. A young boy and two teen-agers had to be pulled from a creek in Darnestown that swelled into a waist-high torrent. And a girl who slipped from a path into Seneca Creek was rescued by several youths using borrowed dog leashes to pull her to safety.

But officials around the area were counting their blessings.

"Amazing, isn't it?" said one Maryland state trooper. "And we were extra lucky -- one of those {tractor-trailers} was supposed to be carrying hazardous materials, and it was just tore up . . . but it was empty."

The curving portion of the Beltway between Connecticut and Georgia avenues, where construction crews are working to add fourth lanes in each direction, looked like a junkyard yesterday after a series of unrelated tractor-trailer accidents there.

One tractor-trailer "climbed the Jersey barrier" separating the through lanes from the construction; another took out part of the wall and scattered the flat-bottomed boats it was carrying across the roadway; and a third was so smashed up that the engine came out of the cab, and the whole carcass had to be "dumped into the middle with the construction," according to state troopers.

The trio of crashes -- each just out of sight around a curve from the others -- wreaked havoc with the morning rush hour, forcing police to close all lanes of the inner loop until about 9:30 and diverting traffic onto the already-jammed Rte. 355 and I-270.

Only three hours later, a trash truck in the middle lane of the outer loop veered out of control and sideswiped a tractor-trailer in the fast lane that skidded into the Jersey barrier and bounced off, jackknifing across the highway. The trash truck rolled over and came to rest on the right shoulder, forcing police to close the same stretch of Beltway, but this time in the westbound lanes, until nearly 3 p.m.

The massive diversion of traffic gave rise to a number of rumors, including stories of dozens of power lines in the Kensington area ripped down by too-tall 18-wheelers, but a Potomac Electric Power Co. spokesman said there were no outages there.

Meanwhile the rain continued, causing scattered flooding that blocked major thoroughfares in the western Virginia and Maryland suburbs, further tangling late afternoon and evening traffic.

Not since the back-to-back snowstorms in January have more motorists had better reason to curse the weather. Rte. 7 around Tysons Corner and Braddock Road between Rtes. 28 and 29 in Fairfax were under water but passable, so that cars inching along threw wings of water into the air.

A slew of roads in the Darnestown region of Montgomery County, including Watkins Mill Road, were washed out in the waning twilight as the Great Seneca Creek attempted to reclaim its watershed. County police said a small bridge carrying Muddy Branch Road south of the Bureau of Standards collapsed, followed by portions of the road itself, which was blocked off between I-270 and Rte. 28.

Still the rain fell. At Dulles International Airport, 3.56 inches fell in the 24 hours ending at 8 last night, and .68 inch at National Airport during the same period.

How long will it last? "How long can you tread water?" joked National Weather Service meteorologist Calvin Meadows.

"It's occasionally moderate, occasionally heavy, but steady," said Meadows. "This is not showers. This is rain."

More seriously, forecasters said that the rain was expected to taper off during the night, with brief viewings of the sun possible today. Showers may return by afternoon.