1
general::
Phrase(s): water over the dam [and] water under the bridge
Fig. past and unchangeable events. • Your quarrel with Lena is water over the dam; now you ought to concentrate on getting along with her. • George and I were friends once, but that’s all water under the bridge now.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, water under the bridge. Something that is over and done with, especially an unfortunate occurrence. For example, Last year's problems with delivery are water over the dam, or Never mind that old quarrel; that's water under the bridge. These metaphoric phrases allude to water that has flowed over a spillway or under a bridge and thus is gone forever. The first term was first recorded in 1797; the variant dates from the late 1800s.
American Heritage Idioms