I'm pretty sure that's not a damn. It appears to be for the construction of the track.
Damn, sure looks like a dam to me!
Seriously though, I'll bet there's a drain to recirculate the water under the bridge and the "dam" is there just to stop whatever water makes it through (and also stop any debris from construction making it into the creek). It doesn't look like it would stop much though. Just next to it, you can also see a wooden makeshift bridge for the construction workers as well.