From the U.S. Forest Service:
Benson Bridge Closure Date(s): Jan 9, 2014 Starting immediately, Benson arch bridge on Larch Mountain trail #441 at the base of Multnomah Falls is closed due to severe damage from a fallen rock. Engineers are working on an inspection. Larch Mountain trail #441 will be closed from Multnomah Falls viewing plaza near Multnomah Falls Lodge to the Gorge #400 trail until further notice.
This isn't the first time a large rock has fallen on the Benson Bridge, but it has been quite a while.
In 1995, a boulder the size of a school bus slid from the face of upper Multnomah Falls and dropped into the upper plunge pool. This created a splash of 70 feet and sent rocks flying over the bridge.
A large wedding party just happened to be on the bridge for photos when the boulder fell and 20 people were injured by rock shrapnel, including the groom. No one had serious injuries though, and the bride and groom's story was featured the next day in the newspaper.