A weird?

The newish road bridge has afforded us a good view of this weir on the River Ness. What is a weir anyway? Is it a kind of man made waterfall? Robert Louis Stevenson wrote an unfinished novel called: 'Weir of Hermiston.' Jack Bruce, the musician, somewhat cheekily named one of his songs: 'Weird of Hermiston,' which I rather like. So, it's a weird!