It's not just old bridges which are going away, it's ferries.
The Louisiana Department of Transportation and Development has announced the Melville Ferry will cease operations in October due to economic circumstances. According to various news stories, it costs LADOTD $100 to $150 per vehicle to operate the ferry, while it charges $1 to use the ferry.
I visited Melville 2 years ago, but was unable to use the ferry since it was a Sunday (it only operates Monday through Friday). I had hoped to go back next Spring and visit it again, perhaps having the opportunity to actually ride on it. If I want to do that, I'll have to drive down there sometime in the next two months, which doesn't appear likely.
Information regarding the history of the ferry is sparse to non-existant. I can tell you it crosses the Atchafalaya River along an old alignment of US 71 (now LA 10) and that the highway is unpaved east of the river. I've contacted LADOTD who said LA 10 would remain in service, as is, for now with signs being posted to redirect traffic around the closed ferry.