Local developers said yesterday that the unexpected resignation of the District official who oversees development along the Anacostia River could slow down the multibillion-dollar initiative and hinder the planning of what should be built around the new baseball stadium in Southeast.

Andrew Altman, chief executive of the Anacostia Waterfront Corp., a quasi-public development entity, said this week that he will leave the organization he helped create as city planning director and led for the past 10 months. He will depart in three weeks for a private-sector job in New York.

Altman, a close adviser to D.C. Mayor Anthony A. Williams (D) and city planning director from 1999 to 2005, had been one of the leading advocates of redevelopment along an area that stretches from the city's Southwest waterfront to the stadium site in Southeast and up the Anacostia River to the Prince George's County line.

"He's been the driving force behind the cultural and economic improvement planned for the Southeast waterfront," said John E. "Chip" Akridge III, a developer who owns land near the stadium site in Southeast. "You have a development corporation that he birthed, set up and was running, and all of a sudden you're taking the captain off the ship. . . . It puts a question mark on the AWC and what if any role it's now going to play going forward."

The Anacostia group has a strong board of directors, including Eric W. Price, the city's former deputy mayor for economic development, and Mitchell N. Schear, a Crystal City-based developer, local developers said. And Stephen Goldsmith, chairman of the corporation's board of directors, said the organization will move quickly to replace Altman.

"We're going to keep our commitments. We're actually going to accelerate our commitments," Goldsmith said. "We've got a whole slew of really important things going on. It can't wait."

While Altman said the corporation is in good condition to weather change, developers said finding the right replacement, promptly, will be critical to progress on Anacostia development.

Raymond A. Ritchey, who runs Boston Properties Inc.'s D.C. office and has built several large office buildings in the city, said that Altman had brought a "forward-thinking approach" to Anacostia development and that "it's now important for city leaders to find somebody who can execute his vision."

The development group was charged with implementing a 20-year plan to transform one of the city's most blighted areas, along the Southwest and Southeast waterfronts, into shops, restaurants, houses and apartments, office buildings, parks and cultural attractions and a new network of bridges and roads. It was also helping decide what should be built around the new baseball stadium at Half and South Capitol streets SE, where there are mostly mechanics' shops, empty lots and abandoned warehouses.

Proposals for how to develop some property near the stadium are due Oct. 21, and developers are wondering if a new leader will be named before then.

"It's a potential for delays," F. Russell Hines, an executive vice president at Monument Realty LLC, said of Altman's resignation and the group's plans around the stadium.

"Things may be slowed down for a while as they try to figure out where to go," he said. "It's important they keep moving and find a new chief executive."

Andrew Altman, right, will resign as chief executive of the Anacostia Waterfront Corp.