Since all other replies are a bit too concise, I'll try and dive deeper (ha ha).
1) Naturally, rains wash out street grime and small pieces of rubbish which may easily bypass the sewage system altogether and end up in the river.
2) Cities have systems in place to treat wastewaters before releasing them into the rivers. However, these systems need to store yet-untreated water in reservoirs, and heavy rain may fill these up. Then, the only solution (if you don't want sewage to blast out sewer holes) is to release the water without treating it. In fact, one of the main upgrades made by Paris for the recent Olympics was a massive, 30-meter-deep new basin [1] built exactly to mitigate that risk.
1) Naturally, rains wash out street grime and small pieces of rubbish which may easily bypass the sewage system altogether and end up in the river.
2) Cities have systems in place to treat wastewaters before releasing them into the rivers. However, these systems need to store yet-untreated water in reservoirs, and heavy rain may fill these up. Then, the only solution (if you don't want sewage to blast out sewer holes) is to release the water without treating it. In fact, one of the main upgrades made by Paris for the recent Olympics was a massive, 30-meter-deep new basin [1] built exactly to mitigate that risk.
[1]: https://www.reuters.com/world/europe/giant-reservoir-ready-s...