[splitdisk]

"Tully said it’s mostly because of the heavy rain, which carries sewage into Lake Ontario and into creeks and rivers."

Yes, this is the problem. Anyone in the GTA can tell you about the wonders of our Combined Sewer systems:

https://en.wikipedia.org/wiki/Combined_sewer

[Gravityloss]

This is like seeing some potentially horrific error handling logic in critical infrastructure. Wait, it is actually that.

The storm drain should have the overflow-to-river mechanism before it's combined with sewage?