> One of the main problems has been the 19th-Century "single system" drainage infrastructure, which unites household waste water from kitchens and toilets with run-off from rain on the street.
Big cities came up with their canalization in the 18th century in two ways: open or closed canals. A closed canal system (with proper tubes) would be safe, no dirt (street dirt, mostly dog shit) could overspill on heavy summer rains into the canals and river, but this would require about 10x larger tubes, just for the few summer days (about 10 days a year). And a normal river with normal oxygen levels can easily fixup itself after such a spill. I.e. a flowing river, no dams near cities. And no industrial sewage directly into the river (which does not happen in civilized countries since the 80ies).
Almost every european city uses an open canal system, where in those few heavy rain days during the summer (and this can likely happen during summer olympics), the canal gates need to be opened so that the street water can overspill into the river, and it mixes with the dirt water from the open canal. So you do have heavy shit pollution in the river for about 3 days. No swimming, bad smell, E.Coli poisoning, rat bacteria (black plague).
So what a few richer cities, or the stupid ones which destroyed the natural cleaning abilities of the river by building dams near their cities for electricity (such as my city Graz, Austria), had to do, was to build such huge overspill cylinders or side canals to keep it from flowing directly into the river, but into their water treatment plant instead.
With more southern cities this would be impossible, as the rainfall is too heavy. E.g. in Houston you'll get a downpoor for 20minutes, with streets and canals flooded, cars need to stop, but then it goes on. In such subtropical cities you cannot think of building closed canal systems or such side spill reservoirs. The peek volume is way too high to be able to control it. And their rivers do contain poisonous snakes and alligators anyway.