Water over the dam (under the bridge)

What does "Water under the bridge" mean?

It means that something is in the past and no longer important.

"My sister and I fought when we were children, but that's water under the bridge."

I think the analogy is that water under the bridge is constantly moving toward the sea. That's a little abstract, so you might also imagine dropping a leaf into the water from atop a bridge. By watching the leaf float down river you'd be witnessing the progress of the water.

WATER OVER THE DAM -- "Something that's done with; an irretrievable situation. An alternative was 'water under the bridge,' as one sees in Richard Sale's 'Passing Strange' : 'That's water under the dam.' 'Bridge,' I said. 'Or water over the dam.'' Either way, flowing water has passed by and will not return." From The Dictionary of Cliches by James Rogers (Ballantine Books, New York, 1985).