This bridge is the fourth to cross the Wild Ammonoosuc River at this site. The first was built in 1810. It was carried away in 1818 by a flood and replaced in the same year. Again, the bridge was destroyed by a flood in 1828. The third bridge was erected in 1829 and remained at the site until 1849. At that time it was dismantled and replaced by the current bridge. This bridge was rebuilt by the state in 1977 at a cost of $34,347. The rebuilding costs were shared by the state and town. This section of the river was used to float logs to the sawmill. Often log jams presented a hazard to the bridge. In one case, dynamite was used to break up a log jam and although the blast was successful, logs had to be removed from the roof of the bridge. The Swiftwater Bridge is listed on the National Register of Historic Places.
For more info (and the site where I lifted the text above), see: http://www.state.nh.us/nhdhr/bridges/p55.html