How catastrophic were the floods of major rivers for the ancient peoples of the civilizations of the Near East, India, and China?

Pretty bad.

Hastinapura was wiped out at least twice within seven generations- so basically a city of around 20K-30K, which was huge by global standards even in the 1600s, basically died every century. Pataliputra was probably destroyed due to a flood in the 7th Century; the alternative- disease- wouldn't have sufficed to destroy a city of what might've been a million people so quickly.

Floods were also what probably destroyed the capital of the Vishnukundina- I forget the name but the place had walls stretching more than ten miles. Some of the floods the city of Kaushambi survived had the waters climbing to almost half the combined height of the ten metre high embankments and eleven metre high walls. In the 10th Century, Markatadeva had over a million cubic metres of clay shifted to protect Cuttack from floods. The largest man-made lakes in history until the completion of Lake Hoover were the ones made by Raja Bhoja- and it's very likely they were created to help regulate the flow of the river Charmanvati- a feat the Republic couldn't emulate until the 80s.

Clearly floods were dangerous enough to warrant all these feats and more.