1890
The Russian River Bridge at Cosmo that was built in 1890 was built on top of the original wooden piers from 1876. This bridge was wood and iron and used a Pratt Truss. The Pratt Truss is a very common design. It was invented in 1844 by Thomas And Caleb Pratt and was a common configuration for railroad bridges as truss bridges moved from wood to metal.
The wooden piers are very visible in this photo.
This photographer stood in the center of the tracks in Cosmo to take this photo (facing east) around the turn of the century. The writing at the top says Camp Six Trescell, Cosmo was also known as Camp Six.
Photo dated September 4, 1908. Note structure on the beach.