Depends on the river. With those Elbe floods that gave upstream so much trouble once the flood reached the tidal area it either hit a low tide which basically just soaks everything up or a high tide, which still was below spring or storm tide levels.

Generally speaking though and tidal systems aside (which arguably are more coastline than rivers) you should always have regions that you are willing to flood. There's plenty of places where seasonal flooding was the norm in the past and they're very valuable biotopes, those should be restored where possible, and even when not flooding a couple of fields and paying out the farmers is rather less of a headache than dealing with uncontrolled flooding. Short video about a flood forest. Bonus: Frozen.

Yes and no. In the north we simply don't build stuff where it could get flooded, or accept that it will be flooded. E.g. Hamburg's Fischmarkt is getting flooded quite regularly (spring/storm tides), no biggie it's built to handle it just let it happen it's not like those cobblestones would be priceless artifacts. Meanwhile, in more mountainous regions places started to think "hmm it was silly of our ancestors to build the village up the hill, we should build new stuff down in the valley by the river" because they hadn't seen a flash flood in a generation.

In short, the issue here is that the people there don't know how (sorry) water comes from above, below, and the sides. Couldn't happen up here that's every other day of the week.