Also, if it's a dam, you could easily have a broad, shallow lake behind it. Then, for greater effect, say they built the wall out of mithril reinforced glass and you also have a conflict, with industrious beaver people petitioning the local elves to dig out the mithril wires.

"Hey, we're only going to remove 25-30% of the mithril TOPS, it's ABSOLUTELY not going to impact the stability of the damn. We've done calculations that back this up! Trust us! What could go wrong? Plus, we'll pay you a 10% tax on all the mithril we dig out of your ancestors' dam! Think of all the trees you could plant with that!"