Virginia Przybylo of Alexandria remembers packing a picnic lunch in the 1930s and walking over to her grandmother's farm at the edge of Mount Vernon Avenue to watch farmers fish and children swim in Four Mile Run.

For the next 50 years, Przybylo saw the whims of the stream's waters dramatically alter the surrounding land, known as Arlandria, which forms the border between Arlington and Alexandria.

First a rural oasis drawing weekend Washingtonians and then an often-flooded commercial and residential lowland between the late 1950s and early 1970s, Arlandria -- now made safe by a federally funded flood control project -- appears headed for days of glory.

"It's going to come more than full swing, it's going to be even better," said Przybylo, who has lived there since the stock market crashed in 1929.

Alexandria City Assessor David Chitlik predicts rapidly rising property values for the portion of the area in his city. "The location is good . . . . The flooding has stopped," he said. Already, a single-family home on Dale Street, representative of many in Arlandria, has jumped in assessed value from $12,000 in 1975 to $52,200 this year, according to city records.

Located within a mile and a half of the Pentagon, Crystal City and National Airport, Arlandria has become a developer's dream. At the same time, it has become a nightmare for the low-income tenants who call it home.

"They want to take this and make it for all the rich people, just like Old Town," said Louise Arnold, the director of the Alexandria United Tenant Organization. "We're just asking the officials not to take our homes."

Largely because the Army Corps of Engineers spent $58 million overhauling Four Mile Run in a project completed in 1980, real estate agents say property values along Arlandria's main thoroughfare, Mount Vernon Avenue, have risen in three years from $3 a square foot to $20.

"It's less expensive and congested than Crystal City and Old Town," said Benjamin C. Zurun, a real estate broker specializing in Arlandria. "We don't have a day go by that people don't come looking for ground along Mount Vernon Avenue."

City officials, pointing to the impending sale and upgrading of 74 percent of the area's rental housing stock and a deluge of small businesses and intimate restaurants relocating in Arlandria, say the neighborhood is fast becoming a trendy hub for professionals and federal employes.

But while most welcome the signs of revival, including wrought-iron Victorian park benches and new street signs, many are concerned that the face lift will force out as many as 3,000 low-income residents and Asian and Hispanic refugees who moved to Arlandria when floods lowered property values and rents.

In Alexandria, where the average rent for a one-bedroom apartment is $495 and the vacancy rate is a scant 1 percent, a new wave of homeless residents could spell a crisis for city officials.

"It would take a Herculean effort to try to relocate those people," said Angus Olson, the director of the city's housing authority.

"There is certainly no place in Alexandria to try to relocate those numbers . . . . And the language barrier is going to be a problem."

"Many of us have been here 25 and 30 years," said Arnold, whose group is fighting what it calls the "rich people's takeover."

Arnold, who lives in a two-bedroom apartment renting for $395 month, said that if developers purchase her Dominions Gardens home with plans for luxury apartments she will have no place to go.

Already investors have bought the 391-unit Presidential Gardens complex and bid on another 1,000 affordable rental units, worrying those residents about being forced out.

"The city's ability to provide tenants with low- and moderate-income housing is severely limited," said Acting City Manager Vola Lawson.

If the sales are between private sellers and purchasers, and the buyer plans to retain the buildings as rental units, Lawson said the city cannot force developers to assist in relocating tenants or set aside affordable apartments.

And that prospect, said Arnold, raises a major question that so far has gone unanswered: "Will there be any place left we can afford?"

City Council members Carlyle C. Ring and Lionel Hope agreed yesterday that although the city would like to see a balance between the upgrading of Arlandria and the retention of affordable housing there, it has little power to ensure it.

Unless developers request tax-exempt financing -- in which case they would be required to set aside 20 percent of the units for low- and moderate-income inhabitants -- or need zoning variances, Ring said the council cannot force new buyers to keep rent increases to a minimum.

"We have no legal recourse," said Hope. "All we can do is keep jawboning -- trying to persuade the developers to work with us."