The region’s turgid rivers ran milk-chocolate brown Friday as they dragged along all the debris that washed from their banks, thick with the nitrogen and phosphorus from spring lawn fertilizing that is toxic to marine plants and animals.

In flood-prone places, the water, driven by several inches of overnight and early morning rainfall, rose up to invade the habitat of humans, flooding the Alexandria waterfront and downtown Annapolis. Roads throughout the region were closed by flooding, and several drivers had to be rescued from stranded vehicles after they attempted to ford pools of standing water.

Rock Creek, the Anacostia and the Potomac all swelled beyond their banks, leaving a muddy residue behind as the water receded. The sun returned to most of the region by afternoon, and a pleasant weekend was forecast, but the Potomac was expected to continue to flush heavy rain from its headwaters.

The rain largely washed out Bike to Work Day on Friday, with many riders abandoning those plans.

“Bike to work day is now boat to work day,” Virginia Del. K. Robert Krupicka Jr., a Democrat from Alexandria, wrote in a Twitter message.

County Executive for Prince George's County, Rushern Baker, surveyed storm damage in Fort Washington that prompted officials to evacuate nearby homes. (Arelis Hernandez/The Washington Post)

Amid concerns that the deluge would cause further instability along Piscataway Drive, in a Fort Washington neighborhood where homes were evacuated earlier this month after the land beneath them became unstable, Prince George’s County officials rushed to the scene. Erosion caused by the rain widened a crack in the street by about four inches, said county spokesman Scott Peterson, but the houses remained.

“One of the biggest fears I had was the fact that I was going to wake up in the morning, come down here and see all this flooded down here,” said County Executive Rushern L. Baker III (D). “So I’m glad we’re able to save these houses here and stop, really, the escalation of this.”

The D.C. region saw between two and four inches of rain, according to The Washington Post’s Capital Weather Gang. In Purcellville, 4.25 inches of rain was recorded. Dulles International Airport had 3.88 inches of rain, and 1.11 inches came down at Reagan National Airport from 6 a.m. to 7 a.m.

The excessive rainfall caused a sanitary sewer overflow in Damascus, Md., as water overwhelmed the Spring Garden Estates Wastewater Pumping Station and caused 45,000 gallons to flow from a nearby manhole, with some debris making it into Little Bennett Creek. The wastewater overflow did not affect drinking water, according to the Washington Suburban Sanitary Commission.

It’s common for hard rains to flood the Alexandria waterfront, and that happened again Friday. As the water rises, residents have come to expect television camera crews to show up at the foot of King or Prince streets. Friday, the water had almost reached Union Street at Prince by about 10 a.m., and then it receded by a dozen or more feet by 11:30 a.m.

Large pools of water stymied some bikers on the Mount Vernon Trail along George Washington Parkway as well as drivers coming off the riverside road onto Memorial Bridge in Virginia. On the opposite side of the Potomac, another regularly flooded road, River Road, was among those closed for a time.

On roads that stayed open, some drivers battled the water, unsuccessfully.

A Loudoun County woman was rescued after she pulled out of her driveway on Tail Race Road and into more water than her vehicle could handle. She took refuge on the hood of her car while rescuers paddled a boat to retrieve her.

In Fairfax County, emergency workers rescued 10 people early in the morning who were trapped in their cars after attempting to drive through flooded areas. Nobody was seriously injured.

The intense rain led to flooding along 45 roads in the county, most of them near creeks and streams that were over capacity, said county spokesman Tony Castrilli. Other roads were shut down because of trees that had been knocked down, he said.

The Maryland State Highway Administration closed about a dozen roads because of the rain, but most of them were in Frederick County and in rural areas, said Dave Buck, a spokesman for the state highway system.

By 12:30 p.m., the region’s roads were clearing up as the skies cleared, and creeks and streams already were receding, Castrilli said.

Dana Hedgpeth, Lynh Bui, Peter Hermann, Arelis R. Hernández, Lori Aratani and Rachel Weiner contributed to this report.