SAN ANTONIO - Meandering for a mile-and-a-half, below the level of downtown streets, the paved paths and landscaped borders of this city's River Walk are a powerful magnet for visitors. So powerful, in fact, that at times walking single file between rows of restaurant tables is the only way to move.

If you find yourself in that situation, you don't want to do what my niece, Alicia, did a few years ago. A jostling crowed forced her to take a step backward at just the wrong place, and she wound up in the river. It's only three feet deep and she was unhurt, but she reported that the muddy river bottom was quite "icky."

Enjoying most of the River Walk doesn't involve such a challenge, and now there's a new, less congested, and more visually interesting stretch of this unique urban amenity that extends almost two miles north of downtown. I suspect that Alicia will want to return after all.

The River Walk's new "Museum Reach," so named because it will eventually link the river to two of the city's major museums, is the first phase of the $279 million San Antonio River Improvement Project, an effort that took more than a decade of planning by civic leaders to realize.

When the extended River Walk is completed in about four years, local officials say, it will be the nation's longest linear park. Sidewalks and bike trails will stretch 13 miles, linking four 18th-century Spanish missions on the city's south side to 112-year-old Brackenridge Park on the north side, where the spring-fed river rises.

The Museum Reach enables passenger-carrying barges to travel north for two miles beyond the downtown section where they operate now. The barge trips include five minutes in each direction in a miniature Panama Canal-style lock that lifts or lowers boats nine feet to compensate for changes in the elevation of the river.

Besides the lock and less congestion, the Museum Reach experience is different in other ways from what happens on the downtown portion of the River Walk. More sunlight reaches most of the new stretch, and even the shady parts of the Museum Reach under bridges are brighter, with eight works of art - all paid for with private donations - along the way.

My favorite installation hangs beneath an Interstate 35 bridge: 25 seven-foot-long, multicolored plastic fish designed by Donald Lipski, an internationally known public artist who lives in Philadelphia.

When you visit here, know that San Antonio is Texas' most popular tourist destination, and catering to visitors one of its leading industries. Thanks to its two main attractions, the Alamo and the River Walk, the city attracts 26 million out-of-town visitors a year, most of them vacationers or conventioneers. That's almost as many leisure and business travelers as Philadelphia sees annually, and Philadelphia's metro area has three times the population.

The River Walk, lined with dozens of restaurants, bars and other shops, is at its most crowded during festivals, such as Fiesta at the end of April, and when there are big sports events at the nearby Alamodome.

But conventioneers, tour groups, and the locals love this place, too, so there's often no way to avoid sharing space with thousands of other people. San Antonio residents urge visitors to just relax and go with the flow.

"Whatever you're here for, your blood pressure goes down," said Dee Dee Poteete, spokeswoman for the San Antonio Convention & Visitors Bureau. "The tie comes off. You step out of the convention and someone puts a margarita in your hand. You start wearing clothes that don't match."

The city has a population that's roughly half Hispanic, 10 percent African American, and the rest "white and other," and Poteete describes the atmosphere as "a blending of cultures . . . you don't get anywhere else. . . . The warmth is genuine, not manufactured. And there's a sense of place here. There are some cities where you don't have that."

There are two good ways to see the River Walk: walking, of course, or floating, on one of the 40-seat barges operated by Rio San Antonio Cruises. Access points from street level to the walk can be found every block or so, both downtown and heading north or south.

The original portion of the walk, where most of the restaurants are, is in a loop off the river's main channel. The paved paths along the banks continue south from downtown for about six blocks, skirting the edge of King William, a leafy neighborhood of restored 19th- and early-20th-century homes.

Heading north from downtown, you're first on a quiet half-mile portion of the River Walk that was finished in 2002, surrounded by hotels and high-rise office buildings and lush, fully mature landscaping. As you walk or ride up the river, most of the tall buildings begin to disappear. You notice three more mid-rise hotels, a couple of restaurants, the Southwest School of Art and Craft, and then, increasingly, older light-industrial buildings.

The Museum Reach and the eight installations of public art start at the Lexington Street bridge (each bridge spanning the river has street names posted). The first is Shimmer Field by British artist Martin Richman, suspended panels of different colors, designed to move gently in the river breeze. Just past the lock is Under the Over Bridge by Mark Schlesinger, a New York painter now living in San Antonio who has experimented with luminous paint that reflects light.

Lipski's installation, titled F.I.S.H., using plastic models of long-eared sunfish, a species native to the river, is a delightful example of how to make the underside of a highway bridge fun.

"That space under a big bridge can be very spooky and intimidating," Lipski told me. "I'm a scuba diver, and whenever you swim around a bridge footing, there are always fish. That was the image that popped into my mind. . . . It seems like the fish are headed north for the summer."

Lipski loves to work in large public spaces, and he said he found this one "spectacular. . . . It's like being in a cathedral."