Louisa Wetherill was told of Rainbow Bridge by a Navajo trader, and she convinced her husband to lead the Cummings-Douglass expedition.
Though its "discovery" by the outside world was only a hundred years ago, American Indians have known about Rainbow Bridge for centuries. It is also quite likely that prospectors, ranchers, and others happened across the bridge in their wanderings.
The story of the official recorded discovery of Rainbow Bridge is an interesting one. Click on the link above to read about this modern day exploration party.
Did You Know?
Navajo Mountain is a laccolith, an igneous mountain which never erupted. Water from the snow on top of the mountain flowed down Bridge Canyon to form Rainbow Bridge.