I always thought the weir was the water behind the dam; the millpond, so to speak. Maybe I was completely mistaken. Doesn't the climax of Thomas Hardy's "Return of the Native" involve some people drowning in a weir? I read that book in high school, many years ago. It was probably the first time--and one of the few times--I've encountered the word "weir." If anyone remembers the novel more clearly than I do, perhaps you could explain exactly what happened.