Having fished in both places, I'd say the water is about as nasty and both have a certain level of, uhh, runoff to deal with
. The "Toilet" is right next to where the water can get flushed out and is closer to cleaner water than the Thames. If the sewage overflow is running for a few days after heavy rains, Prov is the dirtier. If it hasn't been running and the only discharge is the clean exhaust water from the power plant, it's probably cleaner than the Thames.
I'd also say that the fish in both places can look downright nasty although there was a stong find of "Clean & Bright" fish last Feb & March in Providence.
Neither place is a recruiting poster for the Sierra Club...