Well, depending on how much of the mountain was removed, the island could have partly caved in on itself (actually it most likely would cave in a fair bit). Mind you, I don't think I would want to be anywhere near land that had a massive chunk of it simply vanish.
Originally Posted by waldronate
But then there is still the problem of the rivers only flowing into it for a few weeks/months before it filled up and eventually flooded over the entire island. Sucks when reality gets in the way of good fictional ideas.