PROVIDENCE -- They've torn this city apart, but they haven't put it back together yet.

With the possible exception of Portland, Ore., no other American city today is reshaping itself more ambitiously than this semi-abandoned former seaport and manufacturing center.

No city, you could probably add, needed it more.

Maybe the amazing planners of Providence are visionaries, creating the city of the 21st century. Maybe they're fatheads, crafting a future ruin at great expense. It's too soon to tell.

Either way, Providence is a fascinating study. A Bostonian can't hope to understand it very deeply. But you can't help noticing the changes.

Especially the Riverwalk.

The Providence Riverwalk is astounding. A whole new river system cuts through the center of town, complete with embankments, bridges, pedestrian paths and parks. You used to have to look sharp to notice any rivers in Providence. They existed, somewhere, three of them -- the Woonasquatucket and the Moshassuck, merging to form the Providence -- but they'd been largely paved over. Sometimes they were even covered with buildings.

The people of Providence have now ripped off this thick blanket of construction to expose the water. Where they couldn't do that, they simply moved the rivers. That's right: They picked the rivers up like vacuum-cleaner cords and put them down where they wanted them.

As a climax to all this, they built an enormous outdoor terraced park called Waterplace, big enough for a modest production of, say, the opera ''Aida," complete with elephants. The Waterplace amphitheater faces a pool, which, with a couple of barges, could stand in nicely for the Nile. That kind of theatricality pervades the whole Riverwalk.

The sponsors of these astonishing feats are an alliance of local leaders -- public and private, state and city -- sparked by architect Bill Warner. Warner conceived and designed virtually every aspect of the Riverwalk. The money came from Washington. The feds bought the argument that moving all these urban mountains would make the mice of traffic run better. As with Boston's Artery Project, the urban design of a city is being reshaped with federal highway funds.

You can't help admiring the energy that created the Riverwalk. But you can still be nervous about its future.

As it stands, the Riverwalk exhibits all the scary emptiness of a thing half-built. It's eerily dissociated from everything around it. Two dreams are possible about the Riverwalk of the future.

The first is a nightmare: The concrete is heaving and cracking. Weeds and trees are pushing through. The city no longer acid-cleans the graffiti or sweeps up the glass. Rats skitter across the steps of the amphitheater. Homeless guys build nests out of trash. Hollywood cameras show up for a sequel to "Blade Runner."

The other dream is, in part, a dream about San Antonio. The Riverwalk in that city is the example that's cited by everyone in Providence. In this dream, barges and music drift up the Providence waterways, outdoor cafes line the banks and "Aida" is indeed in progress. Joggers and lovers frolic. Best of all, a gingerbread ferry is blowing its whistle. Filled with well-heeled tourists, commuters and excursionists, it is about to depart, from the very heart of Providence, for Newport and Narragansett Bay.

Either dream could become reality. The outcome depends on two fairly mundane things. One is maintenance and the other is commerce. As to commerce, there's a big difference between Providence and San Antonio. In San Antonio, hotels and restaurants belly up to the Riverwalk in a pretty much unbroken line. They feed the waterway with activity and vitality.

Nothing like that can happen in Providence. Too much of the banks of the Riverwalk is already committed to other uses: to a major new artery, Memorial Boulevard, and to other streets, plazas and parklands -- to emptiness, in other words. Some parcels are slated for future development, but maybe not enough to generate a critical mass of activity. The city hasn't even been able to attract a restaurant to fill the space intended for one at Waterplace. And San Antonio is warm: Its deeply shaded Riverwalk is a welcome refuge. Providence is cold, and its Riverwalk is open and exposed.

It's true that Providence's Riverwalk was filled with picnickers and neckers last summer, and you could even spot an occasional canoe on the water. And already the city is programming festive activities for the coming season, including contracts with vendors for food and drink. But it takes more than planned frivolity to make a place like this work. Programming a public space is like feeding a patient through an intravenous tube. If the place doesn't develop a life of its own -- including a commercial life -- the programming will flag once the novelty wears off.

As for maintenance, the problem, of course, is that the great US public -- that's us -- can't be trusted to maintain a pothole, much less a Riverwalk. It's relatively easy for a city to make big capital improvements, especially with federal dollars. That's why cities are always trying to cure their problems with one huge pep pill: an arena, an aquarium, a festival marketplace, a megaplex. It's far harder to maintain such improvements over the years within the constraints of a municipal budget.

And Riverwalk is going to need a lot of maintaining. Which brings us to the architecture.

Bill Warner has designed his bridges and gazebos and terraces in a traditional style. It's a sort of let's-pretend-we're-English-gentry look, a little bit like the magnificent two-century-old streets of College Hill in the nearby East Side. There's nothing wrong with that. But Warner -- and this happens so often today -- hasn't been able to afford the traditional materials or craftsmanship that once went into the making of this kind of architectural form. So the handsomely sculpted caps on the bridge posts, for example, tell a lie. At first you see them as carved in limestone or granite, but they turn out to be cast concrete. They've been made the way you make cupcakes, by pouring mush into a mold.

The result is a loss of any sense of authenticity. Once you realize how it's been constructed, you can't help perceiving the Riverwalk as a stage set. There's no subtlety or variety, no interesting detail. A stone carver with a chisel makes a very different object from what pops out of a concrete mold. Or take the bricks: They're flat, shiny and uniform, a far cry from the richly varied and textured masonry you expect in a traditional civic monument of the kind the Riverwalk pretends to be. Finally, things just don't look durable. And if there's one quality you absolutely must achieve in this kind of traditional architecture, it's a sense of permanence. The message you're trying to broadcast, after all, is a message about the continuity of culture over time.

There's a reason the Riverwalk doesn't look durable: It isn't. Already, there are gaps and irregularities in the concrete, even rust marks where steel reinforcing is staining through. In a New England winter, with its frequent freeze-thaw cycles, this kind of construction requires a lot of care.

Not wanting to bring a parochial Boston view, I talked to several architects more familiar with Providence. All find the Riverwalk's architecture a little ponderous. All suggest something more transparent, like the steel spiderweb bridges of the city's industrial past. Warner has actually produced a couple of these, in the Waterpark area, and they do look better.

He's done some other nice things, too, squeezing what he could from the pathetic budgets we, as a society, now allocate to our public realm. There are handsome brass railings. Some of the retaining walls are built up out of thousands of huge chunks of recycled granite. Interesting little historical markers and street names are set into the concrete.

Riverwalk isn't finished yet. A final stage is due to be complete later this year, including another big public square that seems entirely superfluous to me. The Riverwalk will be dredged, too. At the moment it's so shallow that nothing bigger than a kayak or canoe will float.

Riverwalk may turn out to be wonderful. It may turn out to be dreadful. Its future depends on the response to these issues.

Huge as it is, the Riverwalk is just a chapter in the story of Providence. Bored, perhaps, with moving rivers, Warner has produced a scheme to move an interstate highway. I-195 will be relocated, reconnecting downtown Providence -- "Down City," as it's called -- with the derelict old jewelry district to the south. Warner's plan has been approved and it's moving ahead.

Yet another mammoth project is proposed for the north edge of downtown. This is Providence Place, currently hanging fire while a new governor renegotiates the deal (once again, federal highway dollars are heavily involved). Providence Place is a humongous shopping mall with a million square feet of space, designed to house 150 stores, 5,000 cars and a bus depot. It will also contain a river and, one level above the river, a railroad.

You were reading correctly: An actual river and an actual railroad are proposed to run through the atrium of Providence Place. This isn't just any old railroad, either: It's the East Coast Amtrak line, five tracks wide. The river is the Woonasquatucket, to be fitted out as an extension of the Riverwalk.

A mall with a river and a railroad could become, in the hands of an adventurous developer, a pretty memorable chunk of architecture. But the question of whether Providence Place should be built at all is hotly debated. Some think it will drain commercial activity from the many beautiful old buildings downtown, although there's little left to drain. Still others fear that -- together with an adjacent new Westin hotel and a rather handsome convention center -- it will become an isolated Vatican within the city.

A lot depends on the final design. If Providence Place is open, trusting, and welcoming toward the city, if it reads, from outside, as a street of pedestrian shops instead of a blank fortress to be approached only by car, it could be a plus. If done right, it might help bring the Riverwalk to life. If.

Providence is a paradox. At the very moment when a city is scraping the bottom economically, it often has its best chance for revival, because it's an inexpensive place in which to invest. Providence may be almost there. It may be on the threshold of a new age. It's small, walkable, beautiful and cheap, and the food is terrific. It has lost its port and manufacturers, but it's slowly gaining the industries of today -- medical, tourist, financial.

Most intriguing of all, it's a city filled with students and artists. There are something like 20,000 college and graduate students in a city of 169,000, and all the schools are growing. One of them, Johnson and Wales University, known for culinary and hotel arts, recently purchased the crown jewel of the city's architecture, the Providence Arcade. Together with the Rhode Island School of Design, Johnson and Wales is one reason you may have visited Providence recently for a good dinner in a lively setting. Food and art: Is this the city of the future?

Maybe. Once upon a time, cities were the place where everybody interesting went -- patrons and artists, intellectuals and entrepreneurs, teachers and students. That intimate social city was replaced, during the 19th century, by another city that was essentially a huge pile of capital and labor: a Detroit.

It may turn out that Detroit was just a blip in history. Maybe the city of the next century will again be a smaller place of intense culture, like Florence during the Renaissance or London in the era of Samuel Johnson, a beautiful city of schools and cafes. Like Bologna. Or Heidelberg. Or Savannah, where another expanding art college is buying up another decrepit downtown. Or Providence?