In Perth , Western Australia, where I was born, we had two major water supply reservoirs (well, in the 50s, this was!) - The Canning Dam and the Mundaring Weir. The M.Weir was the one that pumped water to the Eastern Goldfields - a squillion miles away in the desert (C.Y. O'Connor's scheme, built in late 1800s, I think). In the good old days, when it used to rain lots every winter in Perth, Dad used to take us for an annual Sunday drive into "The Hills" to see the first water overflowing at the Dam and then we'd finish the day's outing at the Wier.
But my point is, they are both BIG holders of water - just not sure why one is labeled a dam and one a weir ...