"The avalanche has already started, it is too late for the pebbles to vote" is a line spoken in the TV show Babylon 5, specifically in the first season episode "Believers". It is spoken by the enigmatic Ambassador Kosh, who very rarely speaks at all.
Even out of context, the line is an interesting line, a pithy way to express the futility of many efforts once events have been put in motion. Within the context of the episode, it is much more meaningful, and even more meaningful within the story arc of Babylon 5 at large. Although we can't know that the writers of the episode or even JMS were planning the line as being a portent of things to come, the great care put into foreshadowing on Babylon 5 makes it a very real possibility that it was.
The first thing to look at is the context of the episode. Believers is what is called in B5 terminology a non-arc episode, meaning the episode does not overtly move the action of the series along. The episode instead involves a minor alien race, the Children of Time, who are trying to protect their son from a medical procedure that they consider sacrilege. Not being members of a race that is powerful or important enough to have an ambassador on Babylon 5, they instead approach the major ambassadors, asking for representation. All of the ambassadors refuse them, although Kosh's refusal is, as usual, the most brief and enigmatic.
Since the overall thrust of the episode is the difference between the idiosyncratic beliefs of the Children of Time, and the more powerful belief system of the humans, Kosh might just be observing non-judgmentally that their attempts were futile.
Within the larger context of the arc, I think Kosh was talking about something more than just this case. This is especially true since Kosh's words are usually saved only for a few people, and are usually extremely meaningful. Given that Kosh doesn't even spend much time talking to either Londo Mollari or G'Kar, him speaking an entire sentence to people totally without power can only be taken in the context of something else. I think that context is the upcoming Shadow War, which is the major mover of Babylon 5's story arc. Two episodes after "Believers", the Shadows first appear. I believe that Kosh's reference to the "Avalanche" is the reappearance of the Shadows, and the line is a bit of dramatic irony that both the characters and the viewers will not yet understand. The dramatic irony is especially dramatic and ironical since it is aimed at the Children of Time, who are probably a member of the League of Non-Aligned World, a group of minor alien races that are consistently used as pawns and cannon fodder by many sides throughout the upcoming Shadow War. So in this phrase, Kosh might be communicating that A) He is too busy to pay attention to these people's concerns because B) there are very major events in motion that C) will overwhelm everyone and D) will especially overwhelm the weaker alien races. Quite a big message for such a small phrase.
And of course, Babylon 5 being what it is, the irony of the statement is turned back upon Kosh. In the episode, the commander of Babylon 5 chooses to take the beliefs of the Children of Time seriously, forbidding his medical chief to operate against their will. Later on in the series, the refusal of the Babylon 5 commanders to let the minor alien powers be used as pawns leads the Shadow War to an unforeseen conclusion. In other words, in both the episode and the series as a whole, the pebbles do vote, much against the expectations of either Kosh or the rest of the Vorlon Empire.