Green’s Dictionary of Slang

(orig. W.I.) trousers that are too short and narrow.

1973		E. Bombeck 1 Dec. (synd. col.) When am I going to stop growing? I bought a new pair of pants last week [...] and already they look like flood pants (over the ankle).
1974		H.L. Foster Playin’ the Dozens 186: A youngster makes fun of someone's pants by teasing him about the length of the trousers. This may take the form of referring to someone's pants as Pacific Oceans, floods, or high waters. [. . . .] The implication is that the pants are too short for the youngster's height.
2003		Kansan.com 18 Apr. 🌐 Quillin said capris pants were finally exiting in lieu of flood pants that cut off two inches above the ankle.