cool shots...this is actually one of my favorite drains in the GTA. After a few years I finally went back with Inventor77 to re-shoot the tanks a couple of months ago.
The tank is designed to prevent an overflow of raw sewage from an active sanitary sewer a km or two upstream of the tank, in the event of a major storm. I've never personally seen it full, but it definitely happens. Would be interesting to know the frequency with which it actually overflows into the creek.
I've been meaning to make a dedicated post of photos from my own recent trip, but here are a few teasers
Full set on www.tohellandback.net
or maybe I'll get around to making an actual post one of these days