@tedu well, you got two problems, theres all kind of chemicals which get into the sewer, not just poop. the poop also contains remainders of medicine etc. (like hormones or painkillers). in general stuff you like to be contained.
the other part is that poop is a fertilizer which will fertilize algae. those then use up all the oxygen during the night, when they can't to photosynthesis which produces more oxygen than required. then you have a shitload of dead animals in the water, additionally to the poop.

so generally, it's not something which one wants. usually rivers are more resilient to this than lakes and such because the water gets replaced rather quickly. i think that a river can probably handle this once every decade. although it might be more cheap to just prevent it, because you don't just have a river full of poop but a city :P