Generally, as soon as you start grubbing around in a river, someone from NRW (nee CCW) will come along and try and prosecute you for poisoning fish, plants, algae or whatever else they're especially fond of.
There's a couple of threads on here about panning in mid-Wales if you have a look.
I think the official line is that you shouldn't do it; and that NRW (for which read CCW) have an unpleasant habit of noticing whenever rivers or streams turn a more interesting hue of orange - and then go and track down the perpetrator.
Hello again darkness, my old friend...