Hello. Have you ever fallen victim to either or both of the following mixed metaphors?
"Don’t worry about it—it’s water over the bridge."
"We can’t do anything about the project now—it is water under the dam."
Wikipedia defines the term mixed metaphor in the following manner:
A mixed metaphor is one* that leaps from one identification to a second identification that is inconsistent with the first one. Example: "He stepped up to the plate and grabbed the bull by the horns," where two commonly used metaphoric grounds for highlighting the concept of "taking action" are confused to create a nonsensical image.
* Metaphor (from the Greek: - metaphora, "a transfer") is language that directly compares seemingly unrelated subjects.
Here’s the deal, friends: Water flows under a bridge, not over one (at least in non-catastrophic circumstances).
The GoEnglish.com Web site tells us that the idiom water under the bridge “[refers] to past experiences and conflicts which we have decided to forget. This idiom compares these memories of the past to water which has passing under [the] bridge.”
Similarly, water flows above a dam, at least it is supposed to.
I term these types of English usage glitches "half heard" errors. That is to say, the offending speaker probably heard but only partly internalized colloquial expressions such as "that’s water under the bridge." The net effect of this mistake is a blind parroting of the expression with no conscious thought as to what the phrase actually means.
Another case in point of a "half heard" error:
For all intensive purposes, you are correct.
As the great Moody Blues sang:
Say what you mean
Mean what you say
Say what you mean
Mean what you say
Think about the words
That you’re using
Speak for yourself
Say what’s on your mind
Until next time.