I was cruising the web and came across this site's explanation of the English idioms "water over the dam" and "water under the bridge".
Currently, you have the meaning for these phrases as:
Meaning: You can say a problem or an experience is water under the bridge, or water over the dam, if it happened in the past and it no longer affects the present.
In fact, this meaning is incorrect. The meaning is that while something did happen in the past (and it certainly affects the present!), there's nothing you can do about it. Since you can't do anything about the past, your time is better spent worrying about something else. Just like water that has flowed down the river, over the dam, or under the bridge: it happened, it had effects (maybe even big effects), but there's nothing you can do now except deal with the present situation.
Just my two cents. :)