SEVILLE, SPAIN -- For nearly 2,000 years, dating back to the Roman occupation of Andalusia, this city has been troubled by the severe, periodic flooding of the Guadalquivir River that forms its western border. The navigable river was a blessing, of course. It provided the vital opening to the Atlantic that allowed this inland city to become a great port during the time of Columbus, and after.

But despite the sizable efforts of Spanish civil engineers in this century to control its flow, the river remained "a limitation on the ability of the city to expand," says Jose Benjumea Pino, assistant to the general director of projects and construction for Expo '92. No longer. Nothing better characterizes the immense campaign of public works that accompanied preparations for the world's fair than the reclamation of the Guadalquivir.

Stagnant and dammed since World War II, its banks obscured by railroad tracks, its flow diverted to a nearby canal, the river has been thoroughly transformed. The tracks were removed and replaced by a broad flood wall that does double duty as a public park. The old riverside train station was beautifully rehabilitated for use as an exhibits hall. A handsome new rail terminal was constructed in the eastern part of the city, not far from its tightly packed medieval core. An earthen causeway-dam called the "Chapina plug" was removed, so the river flows freely once more in its original path. Nine mostly spectacular new bridges -- six in the city -- have been completed in the past three years, compared with four in the preceding century and a half.

And the former flood plain has been transformed into the Island of La Cartuja (after a restored 15th-century Carthusian monastery), about 1,100 acres of publicly owned land, this year the location of a world's fair, next year the site of "Cartuja '93," which is what local politicians and planners say when they talk about the reuse of the site. Unlike many previous world's fair grounds, that of Expo '92 is not destined to become an empty lot when the fair is over.

As impressive as it is on its own, Expo '92 has been "just a pretext, an excuse to organize different infrastructures in this region," in the words of an Expo director at a recent press preview of the site. The words are familiar, echoed by all officials of the fair, from Commissioner General Emilio Cassinello Auban to the tour guides. And they seem justified.

The list of infrastructure improvements continues. Among private-sector initiatives it includes the on-time construction of 19 new hotels in and around the city, and the completion of Expo City, a 2,000-unit mid-rise residential development that will become a new suburb after fair employees leave it this fall.

Among public works it includes the construction of a beltway circling the city to provide easy access to Expo's five entrances and extensive parking lots, and, not incidentally, to relieve the old core of some of the pressure of automobile traffic. The ring road connects to an entirely new regional system of limited-access highways, offering speedy passage to and from the resorts of the Costa del Sol and other important points. A new highway to Madrid has been built. A new high-speed train linking Seville with the capital (cutting travel time from six to just under three hours) is set to begin service.

Besides more than tripling the capacity of the old Seville airport (from 1,300 to 4,300 passengers per hour), the new one is beautiful in the bargain. Designed by architect Rafael Moneo, it's austere yet satisfying, a long rectangular structure sheathed in a wonderful golden stone. Its main waiting room, a splendid double-file march of massive stucco-covered piers and high domical vaults, fittingly recalls both Roman and Moorish antecedents while relying on 20th-century engineering.

The exact future of Cartuja Island, after the fair, is a matter of local dispute. Because of its central location, the problem, in Benjumea's view, will be to prevent its "too rapid development, to maintain the purity of the concept."

That concept is to turn the northern portion of the island (not used for Expo) into a metropolitan park with sports facilities; to maintain the eastern portion of the Expo site, including the artificial Lake of Spain, as a cultural-entertainment area encompassing Expo's theme pavilions and its experimental theater, outdoor arena for film and outdoor auditorium for rock concerts and other events; and to transform the area now occupied by the national and corporate pavilions into a research and development center for southern Spain.

To this end the grid of Expo roads was laid out, and the island was equipped with an advanced telecommunications network. Likewise, all of the huge theme pavilions and many others (including Siemens, Fujitsu, Rank Xerox, Canada, Spain, Italy, Andalusia and the plazas of the Americas and of Africa) were constructed for permanence, to be occupied by offices and research facilities. A World Trade Center already is in operation on one edge of the site. Nearby is rising a prepossessing, Louis Kahn-ish circular edifice for offices of the Andalusian regional government.

A special problem not mentioned by Benjumea will be to prevent the spread of dreary high-rise suburbs that already surround the old city -- the new roads are an inducement to replicate this sorry standard. Surely there are better patterns. But, still, this is a formidable performance by the architects, planners, engineers, financiers and public officials of the new Spain. And nothing quite symbolizes the optimism of the endeavor like the new bridges, all strikingly modern and lovely.

The Alamillo Bridge is astonishing, a 200-meter span supported by cables from a single, angled pillar 140 meters high. Designed by Catalonian engineer Santiago Calatrava, it's sometimes called "the harp bridge" because of its highly original configuration. In a site not lacking in vertical monuments, the all-white Alamillo pillar jutting sharply into an all-blue sky is the most unforgettable. Seen lit up at night from one of the new roads west of the city, it rivals the 800-year-old Moorish-Catholic Giralda tower as a sign of Seville.