Literal translation
Water will pass by at the river.
Meaning
a lot of water will flow under the bridge
Explanation
The expression pictures the passage of time through the steady flow of a river: water that flows past and never returns, a symbol of time moving forward with no way back. The construction n'en passara, water will pass, places the event in an imprecise but certain future: something will happen, but only after enough time has elapsed for much water to have flowed by. It corresponds exactly to the equivalent saying about water flowing under bridges, sharing both the image and the function. It is used to temper an expectation, to recall that a hoped-for or feared outcome is not imminent, or to underline, with a quiet fatalism, the inevitable slowness of certain changes. The register is ordinary, without any particular weight or lightness: it is a neutral, imagistic way of speaking about long stretches of time, often used regarding a project, a reconciliation, or an event whose timing remains uncertain.