Why did Gandalf let go and fall off the cliff?

well, they fell into water. anyways, if you were a couple thousand years old, just had a fight with someone as strong or stronger than you, and were holding on to the egde of a bridge that is falling apart, how long do you think you would stay on? i don't think he sacrificed himself, because if he had gotten out of moria, the fellowship would have known what to do, and may not have broken apart. plus, he was the leader, so why would he leave them intentionally? i don't know, just my thoughts. it worked out for the better anyways...