Engineer James B. Marsh of Des Moines patented the rainbow or Marsh arch design in 1912. Although expensive to construct, this graceful and innovative concrete and steel design was used throughout the country. When it was built to bridge the South Platte River, this single Colorado example of the type was said to be the longest such bridge in the world. Its eleven spans arch gracefully over the roadbed from below-grade piers. The three southern spans have been reinforced by steel tension rods, and a second bridge was built in 1963 to relieve traffic volume on the narrow roadbed. Now designated a Civil Engineering Landmark, the bridge has been converted to a pedestrian crossing with the original cast iron lamps recast and reinstalled in 1995.