Channel Properties held a “bottoming out” ceremony Friday for the Riverfront, an ambitious mixed-use development with aspirations of redefining downtown Napa’s riverfront.

From what is now an enormous 25-foot-deep hole on Main Street, stretching from Third Street to Fifth Street, will emerge three and four stories of residential condominiums, specialty stores, restaurants and offices.

“Our future is today. It’s here,” said Cassandra Walker, the city’s economic development manager, at ceremonies attended by some 50 people.

Mike DeSimoni Sr., a longtime Napa resident with a passion for downtown, said his family’s project would accelerate downtown’s makeover as place for dining, shopping and recreation.

Located next to a portion of the river trail that will ultimately extend seven miles, including a broad promenade through downtown, the Riverfront will help Napa achieve its tourist potential, he said.

“This is going to be a destination. This will be a place that people want to see, a place where you can walk with your kids,” DeSimoni said.

This is likely Napa’s most expensive private development, with an estimated cost of $72 million. The 50 condominiums will sell for an average of $1 million, DeSimoni said.

DeSimoni said 178 potential buyers had already expressed interest in buying third- and fourth-floor condos that won’t be available until late in 2008.

“This is a project that will kick off the revitalization of downtown Napa,” said Margaret Hager of the Mechanics Bank, a lender to the project. Many potential buyers are older Bay Area residents who would like a second home in the Napa Valley, she said.

The Riverfront will have nearly an acre of retail and restaurant space, with premium rents in the vicinity of $5 a square foot, nearly 50 percent more than downtown’s current top rents.

One of three restaurant spaces has already been leased to Emporio Rulli Gran Caffé & Ristorante, which has an operation on San Francisco’s Marina District, said Craig Semmelmeyer of Main Street Property Services, the retail leasing agent.

In addition, the Riverfront will have several smaller eating spaces as well as a coffee shop. The coffee space has been leased to a highly-regarded operator whose name will be released shortly, Semmelmeyer said.

The project’s location on the Napa River and the downtown river promenade will be a major plus in attracting both tenants and customers, Semmelmeyer said.

Four blocks of promenade, from south of Napa Mill to north of Downtown Joe’s, are scheduled to be open to the public early next year.

Eventually, a river trail will run the length of the city. It will connect such emerging downtown attractions as the Westin Verasa hotel, the planned Ritz-Carlton hotel and the nearly finished Oxbow Public Market, as well as two established tourist draws, the Napa Valley Opera House and Copia.

The Riverfront will have a two-story garage with more than 200 spaces to accommodate residents and workers in 30,000 square feet of Class A office space.

The garage and buildings will rest inside a concrete “bath tub” with a dewatering system to keep ground water out, said Dave Johnson, whose firm, Johnson Lyman Architects, designed the project.

Piles were pushed deep into the ground to anchor the project and counteract the buoyancy of the ground water, Johnson said.

Asked to describe the project’s architecture, Johnson said it captured the historical flavor of the Napa Valley as well as the “Old World.”

The Riverfront is also buying 50 spaces in the new city-county garage that is the preliminary stages of construction on the north side of Main, behind the jail and county offices.

Channel Properties is an off-shoot of the DeSimoni family’s main business, Channel Lumber in Richmond. Channel Properties also owns the Winship Building at Main and First, which underwent restoration several years ago, the Alexandria Square office-restaurant building at Main and Second.