Work to replace the Cradock Bridge is expected to begin late next year or in 2013, State Representative Paul Donato said Wednesday.

Design work on the project is about 50 percent complete, Donato said. The project aims for 90 percent design completion by in spring 2012, with construction in the fall 2012 or spring 2013, he said.

The bridge, which carries Main Street over the Mystic River near Medford Square, has been falling apart for years, Donato said, but the need to replace it became apparent with the torrential rain in March 2010, Donato said.

Debris floating down the Mystic got caught on concrete barriers beneath the bridge, which were once needed for locks the bridge used to have on its underbelly. The debris contributed to severe flooding in Medford, Arlington, Winchester and Cambridge, Donato said.

The project is expected to cost $2.5 million. About $2 million in financing to replace the bridge was secured in the state's transit bond bill, passed last year, Donato said. Another $500,000 had previously come from the state budget for design work several years ago, he said.

The original bridge was built 1637, rebuilt in 1880 and again in 1909, according to a Medford Historical Society sign on the bridge. The original wooden drawbridge was the first toll bridge in New England.

Mayor Michael McGlynn said he was pleased to learn there were plans to replace the bridge.

"The bridge is deteriorating," McGlynn said. "It has to be done."

He also said he wanted the project to include space for a walking path beneath the bridge along the bank of the river.