This is a commonly used malaphor, blending two idioms – “water under the bridge” and “water over the dam” – that contain the same word (water) and have the same meaning (past and unchangeable events). Add two words that both describe direction (over and under), and two structures that are in or over water (dams and bridges) and you get a subtle mix-up. I have posted a few similar malaphors – see “that’s water over the bridge” (8/4/12) and “that’s just blood under the bridge” (11/9/12). Many thanks to John Costello for this one.
