Swimming in the Hudson River used to be followed by a trip to the hospital for a tetanus shot. Alan Zollner and his club think it?s time for swimmers to take the river back.

?The river is getting cleaner,? said the president of the River Pool at Beacon, a nonprofit project headed by the Beacon Sloop Club in Beacon, N.Y., 60 miles north of New York.

The project a $100,000 floating circle of interconnected modular fiberglass and a webbed nylon bottom allows water, light and fish to flow through the pool. A smaller prototype will be installed by June.

Though the water in many rivers is getting cleaner thanks to provisions such as the Clean Water Act, the bottom sediment is still quite toxic. Also, environmental groups worry about upsetting delicate ecosystems. Floating river pools solve both problems with a design that keeps swimmers away from that area and safe from sweeping currents.

The idea of floating pools conjures turn-of-the-century images of wood bathhouses and river docks filled with swimmers. These historic flow-through bathhouses are where architect Meta Brunzema first got the idea.

In fact, environmentalists were the ones who opposed the idea, recalled the owner of New York-based Meta Brunzema Architects, because such a structure would create shading and interfere with the river?s ecosystem. With that in mind, she set to work designing a transparent pool, now being adopted by the River Pool at Beacon. She said there?s huge interest for her design along the Hudson River.

Because fish can swim through, Brunzema expects the pool to be more than a place to swim. It will be an environmental classroom, too.

Other cities now are considering similar projects. The Charles River Conservancy in Cambridge, Mass., is working with the Massachusetts Institute of Technology to develop a structure as well. Officials are conducting site analysis and exploring the possibility of swimming without touching the toxic sediments, explained Renata von Tscharner, president of the Conservancy.

?The Charles is the front yard ? to about half a million people who live and work within walking distance,? she said. ?Swimming in the Charles River in the ?50s was so popular, they even had lights to illuminate the beaches at night.?

Von Tscharner cites the extensive European research conducted for the River Rhine (Germany), the Limot in Zurich, Switzerland, and the Lake of Zurich. In Copenhagen, Denmark, the Copencabana turned the industrial harbor into the city?s social center. The 1,600-square-meter harbor swimming area is divided into three pools, with a beach, palm trees and a Moroccan vibe.

In Philadelphia, the William Penn Foundation is working toward a grant to conduct a feasibility study for a river pool in the Schuylkill River, said Brent Thompson, director of communications.

And in Chicago, river fans are hoping to bring swimming back as well, said Margaret Frisbie, executive director of the Friends of the Chicago River. While a floating pool would protect swimmers from the many tourists boats, Frisbie said the project is years away. ?We are slightly behind our Eastern counterparts,? she noted.

Chicago, too, once had its share of recreational river swimming. Even with its stretch of lakefront beaches, many residents cannot access them. But as Frisbie said, ?The river is a lot closer and floats through a lot of communities. It opens up the possibility of swimming.?

There?s no doubt the river pool concept is catching on, experts say. ?It?s bringing more life and people to the river,? Zollner said. ?The more people appreciate this natural resource, the more they will be able to care for it and protect it.?