What's the origin of "water under the bridge"?

I could not find earlier mention in English, but in French several expressions using the component water under the bridge with the acception of time passing only in one direction can be found in the sayings:

"Laisser passer l'eau sous les ponts"

(to let water flow under the bridge, meaning to let it go)

and

" Il passera bien de l'eau sous les ponts"

(There will be lot of water flowing under the bridge meaning it'll happen in a long time)

Both expressiosn can be found in the 18th century Dictionnaire de l'académie française http://books.google.fr

The idiom, water under the bridge is akin to another expression What's done is done, which means it is too late and pointless to change the past, too much water has flowed; i.e. time stands still for no one.

If you have ever tried to stop the flow of a river, (and who hasn't?) the comparison and meaning of water under the bridge becomes clearer. The person is saying they have moved forward with their life, the matter is now unimportant and not worthy of further consideration. As the river flows on, so does life. Since we often associate running water with rivers, and where there are rivers there are usually bridges, water under the bridge is similar to a fixed phrase, or chunk of language (you also have the expression bridge over water). One could argue that the bridge represents the permanent present, while the river represents life and time which marches inexorably forward.

The earliest example I found is in 1934. A song entitled Water Under the Bridge , written by Paul Francis Webster, Lew Pollack and performed by Fred Waring. The title was suggested by Ed Sullivan. The first line of the chorus begins

We kissed and love flowed thru my heart like the water under the bridge

Based only on this snippet, (in vain I tried to find the lyrics online), it appears the meaning of the idiom was not familiar at the time.

But it might explain its appearance in 1935.

"The Sugar Bulletin, 1935-1936"
_{NEW ORLEANS, LA., OCTOBER 1, 1935}

Hardly the stuff of poetry, but earlier instances I did not find.