NEW YORK — Steel and concrete panels that were once part of a mighty bridge that carried 50 million vehicles a year across the Hudson River north of New York City will find new life spanning streams along sleepy country roads.

With traffic now whizzing across its shiny replacement, the 61-year-old Tappan Zee Bridge is being painstakingly dismantled in a process that will stretch into 2019. Barges haul sections upriver to Albany and downriver to Perth Amboy, N.J., where ground-up concrete will be sold for highway construction and steel will be melted down and recycled.

Some pieces of the old bridge will escape the crushers and furnaces and be trucked to upstate towns looking to save thousands of dollars on their own bridge projects.

Essex County’s plans include using two panels to span a creek along a gravel road in Ironville, a national historic district near Lake Champlain that bills itself as the ‘‘Birthplace of the Electric Age.’’