At first I thought that this was a railroad bridge but after I saw the next 'overpass', I had to think that it could be a flume for larger logs....see below.

Finally I saw another overpass and it was a Flume. (DUH-! We are in logging country). The definition of a flume is: A narrow gorge with usually a stream flowing through it or an artificial channel or chute carrying a stream of water, as for furnishing power or conveying logs.