Regarding the goml signs about dangerous weirs. If I recall correctly these went up shortly after a local man drowned in a canoe downstream from Downton Castle. So I'm guessing it is a response to this trying to discourage potential canoeists from entering the river rather than suggesting there is a weir right there. Perhaps.
Also another completely different point, I've seen Criftins Ford bridge referred to as Black Bridge on other maps. In fact the house next to it is called Black Bridge house ( or cottage or something ) and was it originally a stone arch bridge before the mid nineties rebuild? I can't find any old photos of it online.