Michael J. DeGeorge, a spokesman for Mayor Byron W. Brown, said the city began the work Monday because forecasts for the week ahead call for fluctuating temperatures that could cause flooding.

Crews said the thickness of the ice on the river ranged from 6 to 10 inches.

Pittsburgh's recent streak of frigid temperatures has caused the rivers to freeze, raising questions about whether the boat could be damaged before it is ever placed into service. "We do check the boat out periodically. It's been checked about three to four times this week," Pittsburgh Fire Chief Darryl Jones said. "One of my assistant chiefs goes down there, starts the boat, knocks some of the ice off it as best he can, makes sure all the systems are working on it.