I did not expect the definitions of pier and bridge to overlap so much!

I don't have a good source and its not clear to me if finding one would clear up bridge vs pier, but, I was once told this tower in the lower reservoir was for taking water out. I would think this was an assumption on their part as it seems they are a common structure in reservoirs.

https://wiki.openstreetmap.org/wiki/Tag:tower:type%3Dintake ("often accessible via a bridge-way") 
https://en.wikipedia.org/wiki/Intake_tower.