I remember reading that Ghengis Khan had a river's course shifted to pass over the city he had conquered to show disrespect towards the fallen enemies.
Is that true, and was river course changing ever a thing in warfare?
Changing the flow of waterways was done several times during the Eighty Years' War between Habsburg Spain and the Dutch rebels. The first major instance was the flooding of dikes and sluices around Amsterdam when Farnese's army approached.
Another major instance was during the Rampjaar in 1672, historically the worst year in Dutch history. An allied army of France, Cologne, and Munster bypassed fortifications around the Scheldt and attacked from the Rhine area. The Dutch army was stunned and the dikes and sluices were flooded to impede the progress of the allies.
For a long time the Dutch relied on the waterveste system, where they would flood the dikes and sluices. There are two lines that are still well known today.