The Dimensions of Rainbow Bridge

by Jay H. Wilbur
November, 2007

The dimensions of Rainbow Bridge (NABSQNO 12-503108-4103439) currently accepted by the National Park Service (NPS) are based on measurements made in 1974 by the Bureau of Reclamation (BOR). These include a value of 275 feet for the span and 246 feet for the height.

In April 2007, with the permission of a NPS ranger at the site, I took precision laser measurements of the span and height of Rainbow Bridge using the NABS standard definitions of these dimensions. Measurements were taken from a series of locations at and near the observation area designated by the NPS for visitors. Because I was not allowed to access the area directly underneath the arch, these measurements were not as accurate as would otherwise have been possible.

Subsequently, I sought formal permission from the NPS (Glen Canyon National Recreation Area) to access the area under the arch for a more precise measurement. This permission was denied in October 2007. Consequently, I am publishing my April results as the best available measurements of the arch.

As with any natural arch, a key to applying the NABS standard dimensions to Rainbow Bridge is to properly select the right endpoints for the span. These are shown in the accompanying graphic below. This photo was taken several years ago when Lake Powell was near full pool level. The measurements reported here were taken at a time (April 2007) when the lake had receded about a mile from the arch and the bottom of the streambed under the arch was clearly visible.

Photo of Rainbow Bridge showing the points measured to determine the span. The horizontal white line at the bottom represents the 1974 Bureau of Reclamation measurement of 275 feet.

There is a pile of mixed soil and boulders at the inside of the northern abutment that must be properly considered when determining the northern endpoint of the span. In accordance with the standard, soil must be considered to be part of the base of the arch while loose boulders are ignored. This was done to arrive at point N in the graphic.

From the observation area, it could not be determined visually which of points S and S’ (shown in the graphic) was the correct point to measure. Therefore, both were measured. It turns out that the horizontal distance between N and S' is shorter than the horizontal distance between N and S, and therefore point S is the correct southern endpoint for the span.

My results are that the span of Rainbow Bridge is 234 feet plus or minus 2 feet. The height is 245 feet plus or minus 2 feet.

My result for the height is very consistent with the result obtained by the BOR in 1974. My result for the span is quite different. The horizontal line at the bottom of the graphic depicts a distance of 275 feet in the scale of the photo. Based on this, I believe that the BOR did not measure the true span of the arch, but rather measured the extent of the arch.

The true span of the arch, in compliance with the NABS standard definitions, is only 234 feet. This is the value that should be used in comparing the size of Rainbow Bridge with other natural arches. Clearly, Rainbow Bridge must move down in rank from its previous third-ranked position on the list of largest natural arches. Furthermore, Rainbow Bridge can no longer be considered the largest natural bridge in the world using any rigorously defined criteria.