The Waldo Bridge is a bridge over Dry Creek at Waldo Junction, located in the midst of Spenceville State Wildlife Area in Yuba County. It is located on Waldo Road, which happens to be a gravel/dirt road. If you go there, be prepared to travel over some of the most pot-holed roads in Yuba County. The closest good road is Highway 20 between Marysville and Grass Valley. You get off of Highway 20 onto Smartville Road (toward the "Grass Valley Gate" of Beale Air Force Base) and then branch off to dirt Waldo Road. But the trip is well worth it, as it is a beautiful and unspoiled area.
The Waldo Bridge is a century old, having been built in 1901 to serve the now-vanished towns of Waldo and Spenceville. It is a metal truss bridge that still has its original wooden planking for the roadway. It's a narrow, one-lane bridge, but you won't get a chance to use it for awhile. It's been closed for repairs since early in 2009. You can still bike and walk across it though.