Is a dam a boon or a bane give one word answer?

Answer 1

No.

That was your one-word answer, but perhaps now that you've seen it you would prefer that I should explain it a bit. I cannot give a one word answer, because it is neither of those things to such an overwhelming extent that it can be fully categorized as either "boon" or "bane". Context is extremely important here, and there are both benefits and drawbacks to a dam; they need to be carefully weighed against each other to determine whether building a dam at a particular site would be, overall, a gain or loss.