This doesn’t entirely disambiguate OP’s sentence - he’s tying the lack of incision to no longer having untreated wastewater. Is that to say that treated wastewater would somehow cause incision, or just that the untreated wastewater increases volume and thus erosion?

Generally speaking events that overwhelm wastewater/stormwater combined systems are large storms, so if the decision is made to release untreated water you're probably releasing a lot of it all at once from whatever few outlets you have.

Maybe that's what causes incision? But hydrology isn't my area of expertise.