Improvements to historic Rainbow Bridge on Idaho 55 north of Smiths Ferry were completed last week, the Idaho Transportation Department announced.

In 2006, work was done on the arch piers, floor beams, deck joints and columns of the bridge, which crosses the North Fork of the Payette River. Construction crews replaced the bridge's rails this spring and summer.

The project's main goals were to preserve the historical quality of the bridge and ensure traffic safety during construction, an ITD news release said. Exposure to the elements over the years caused the bridge to deteriorate. The structure's rails were severely pockmarked, and the deck was uneven.

Rainbow Bridge is the largest single-span concrete arch structure in Idaho. Built in 1933, it remains today as a major achievement reflecting leading-edge bridge engineering at the time and exemplifies a conscious effort to meld a modern structure with a picturesque natural setting, the release said.

Listed on the National Register of Historic Places, the open-spandrel concrete arches reach 410 feet across the North Fork.

Designed by Charles A. Kyle, the first chief bridge engineer for the then Idaho Department of Highways, construction began in July 1933 and was completed in December of the same year.