I camp here many times, fishing under the bridge and camping nearby. This is a railroad bridge with 12x12 or so uprights on sturdy concrete footings nearly every 3-5 foot and a ton of bracing. It is NOT a turntable it is just a bridge that turns for the canyon terrain. A turntable would be to turn an engine around this is not that. In fact the railroad was used to haul men and supplies into the powerplant and dam worksite as well as haul pennstock tunnel rock out. The turning of the small 0-4-0 tank type engine was done at times with the powerhouse crane, a picture which reportedly is still in the powerhouse. The bridge sounds wonderful crossing it and is a gem of the area.
Strongly agree that it appears to be an old railroad turntable. Compare this bridge to a bridge that I have proof was a turntable (because of the text and patents listed on the builder plaque) http://www.historicbridges.org/michigan/turntable/index.htm
This bridge is a few miles up a side canyon of the North Fork of the Feather River canyon. The main canyon holds the Union Pacific RR line which was opened in 1909 and operates to this day.
It looks like the main span may have an old turntable bridge. Was this an old railroad grade converted to a road?
I am interested in responses to this deck plate girder design. I would describe it as an inverted camelback deck plate girder. I have not seen another of this type.