The final blow in an unprecedented series of floods across southeast Louisiana struck the expensive suburbs of St. Tammany Parish (County) today amid blue skies and balmy weather.

The swollen waters of the Pearl River were cascading over the broad lanes of Interstate 10 this afternoon, obliterating a mile-long stretch of the highway and rolling indiscriminately through trailer parks and river camps into houses priced at $200,000 and up.

By early afternoon, perhaps 3,500 people had already left their houses, Capt. Bill Dobson of the sheriff's office estimated. But the water was still rising, and the homes of as many as 25,000 people were threatened, according to civil defense officials.

In Cross Gate, where houses sell for up to $100,000, several were engulfed by three feet of water while neighbors up the block watched the muddy waters of the West Pearl River creep up the street.

The banks of the West Pearl itself were more than a mile away, on the other side of a swampy wood of pine and gum trees.

Officials from the Army Corps of Engineers told reporters earlier in the day that they have yet to calculate the magnitude of the last week's flooding in the state with any precision, but said the odds against it happening in any given year were much higher than 100 to 1. The so-called 100-year flooding that occurred in 1977 doesn't compare with it, they said.

Officials said flood waters from the Amite, Comite, Tickfaw and Tchefuncte Rivers were receding, but not fast enough to suit thousands of property owners. According to Col. Robert Lee of the New Orleans District of the U.S. Army Corps of Engineers, "all those rivers have met or exceeded their previous records."

At the height of the storms, which killed a total of 13 people in Mississippi, Louisiana and Tennessee, 27,000 people were forced from their homes. Thousands remained out of their homes today in Mississippi. Officials said damage in Mississippi and Louisiana could reach $350 million.

In Louisiana, where four lives have been lost so far, Gov. David C. Treen has asked President Reagan to declare 16 parishes federal disaster areas, making residents eligible for low-interest repair loans.

An inspection team from the Federal Emergency Management Agency is scheduled to arrive in Louisiana Monday.

While the rains appear to be over for the most part, the search for scapegoats is just beginning.

Here in Slidell and the handsome subdivisions east of the city, home for thousands of commuters to New Orleans, many people seem to think the West Pearl should have been dredged to prevent flooding. Others suggest that Interstate 10, the big east-west highway that has previously served as a protective levee, should never have been built, at least not without drainage pipes underneath it.

Still others fault the developers and home builders who put up so many dream houses on low-lying land with assurances to buyers that flooding would not occur here.

The Pearl and its tributaries, it is plain, don't like to be second-guessed. The river, which begins in Mississippi, ends in a crescendo here. The West Pearl flows closest to Slidell. The East Pearl amounts to the Mississippi State Line. The Bogue Chitto and other streams amount to partners in crime who join the plot somewhere below Bogalusa.

This morning, at a gauge a few miles to the north, the muddy brew crested at 21.18 feet, a record according to the National Weather Service. Dave Barnes, the weather service's chief meteorologist at Slidell, said the water would remain at hazardous levels here for another 48 hours.

Barnes had an interest in being especially accurate in his predictions. His own home, he said, was under three feet of water this morning.

A quick ride in an Army helicopter at midafternoon showed that the worst flooding then was to the north of Interstate 10. There, one of the highest-priced houses in town, in a subdivision called Magnolia Forest, used to have a protective levee encircling it. The levee was breached today.

One homeowner took it philosophically while she worked on sandbagging her house as water rushed across her front lawn. She said she is from St. Louis and, she reasoned, "up there you've got tornadoes. Down here, you've got floods." Her business? Real estate, she told a reporter.

Others were a bit more annoyed. "I'm resentful, very angry," householder Margaret Elliott told a TV crew.

"When we bought this house," Hickory Hills homeowner Roger Vendt told the same crew, "the people said you might get a little water on the ground."

Asked why taxpayers from the rest of the country should subsidize development on flood-prone areas by providing disaster relief, Gov. Treen declared at a midday press conference in Baton Rouge that he thought it essential.

"We've got to house the population," he said. "We're producing energy offshore for the whole nation."