A mother and her young daughter were killed by apparent electrical shocks Saturday afternoon in the Chevy Chase area of Montgomery County, as heavy rain produced widespread street and property flooding, authorities said.

One person died at Lake Linganore in Frederick County, Md., in a boating accident that officials said was due at least in part to the day’s storms.

The victim was flung into the water after a boat was swept over a dam, said Lt. Jason Null of the Frederick County sheriff’s office. Several other people who had been in the boat were rescued. A fatal car crash in Montgomery may also have been storm-related, authorities said.

Trees toppled and fell onto roads and houses. Thousands of people lost electricity, and heavy damage was reported in parts of Southern Maryland, where residents described what they thought was a possible tornado.

In the Chevy Chase incident, a 29-year-old woman and her 4-year-old child were found unconscious in an outside stairwell leading to the basement of their house in the 8800 block of Connecticut Avenue, county police said.

A police spokesman said a portable sump pump was in standing water near the victims, and detectives were looking into whether the water had become electrically charged.

One person was killed in a two-car crash late Saturday afternoon in the Gaithersburg/Derwood area of Montgomery County, authorities said. The crash occurred about 5:30 p.m. at Shady Grove Road and Epsilon Drive. Shady Grove Road was closed while police investigated. It wasn’t immediately clear if the crash was storm-related.

The possible tornado was reported in the Clements area of St. Mary’s County. According to Chief John Nelson of the Seventh District Volunteer Fire Department, witnesses said a twister had torn across the area, felling as many as 75 trees. Also damaged were three homes and 10 other structures.

Almost three inches of rain fell in parts of the Washington region, sending water across low-lying streets, roads and trails from Stafford County in Virginia to Baltimore County in Maryland.

Water filled streets in the Newburg area of Charles County and in Fredericksburg, Va., where part of the roof of a downtown tavern apparently caved in.

Water streamed across part of the Washington & Old Dominion Trail in Northern Virginia and rose to car-bumper level in Essex, Md. One person was rescued from a car trapped in high water on Riffleford Road in Montgomery County.

A tree fell onto a house on Winding Waye Lane, in the Glenmont area of Montgomery. A woman trapped in the house was rescued.

By 10 p.m., 2.75 inches of rain had fallen at the official measuring station for Washington at Reagan National Airport.. That set a record for the date, eclipsing an 1883 mark

Saturday’s rain, according to the Post’s Capital Weather Gang, brought the month’s total to 11.93 inches, the second most in June in Washington since record keeping began in 1871. .