I second the notion that your neighbor has some issues. And, I also second the opinion that you're not obligated to wait until she has inundated your property to take legal action. It would seem to me - not a lawyer - that it's pretty demonstrable if her dam really will flood your lot, and that, if that's established, it would be pretty open and shut to legally force her to remove it or alter it to the point that it won't.

There's a proverb in the Bible that says not to throw pearls before swine. In this case your attempts at civility and decency constitute the pearls. Time to take the gloves off - she won't respond to anything else.