This bridge over the Glenquarry Cut forms part of the Shoalhaven Scheme, which provides water management and electricity generation. Specifically, the Cut joins the Wingecarribee & Wollondilly system to the Nepean system, allowing the generally higher-rainfall Southern Highlands to provide water to Sydney or Wollongong in times of need. Here you can see the Control Structure which regulates how much water, if any, passes through the Cut.
More info:
http://peonyden.blogspot.com.au/2007/06/shoalhaven-transfers-via-glenquarry-cut.html
http://en.wikipedia.org/wiki/Shoalhaven_Scheme
I'm placing this one and some others as part of a leadup to the "31 Days of Geocaching" in August 2013, so that locals who have already found everything nearby have a few more to help complete their spree. I'm hoping some other locals do the same so there's some to be found by me too! :)
** Congrats to Mr. Rick for FTF! **