Game concept
Flood
The game also links it as Floods, Flooded.
From the game's encyclopedia
A Flood is a Natural Disaster which causes growing damage in a large number of Counties.
Floods can only occur along the banks of major rivers.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Flood is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Flood is a Natural Disaster which causes growing damage in a large number of Counties." It is part of Disaster. The game also links it as Floods and Flooded. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.