I have no idea what the scale is, but I could actually understand a ban on gold dredging. I suppose you could control it by issuing a very few permits, but that would get ugly. At the same time, a bunch of folks digging up stream beds and dumping it back in would necessarily fuck up the water quality. Sure, it would recover but would you want your kids to play in a clear stream or a murky one?
Maybe you should look at how gold dredging works first before commenting on it. First of all, small time prospectors, by dredging, actually clean up the rivers. It takes out all the old mercury, lead shot and whatever other harmful metals that were placed in the stream, usually prior to the 1920's. Secondly, small time dredgers are only doing to the stream what the stream does to itself which is move the overburden constantly. This takes old rocks from the bottom and moves them to the top. This allows for more gravel beds in the stream which is actually where salmon lay there eggs. Lastly, these dredges do not produce big plumes of sediment. They are designed to bring the rocks from the bottom of the stream and place them back to bottom. The sediment does not come from the stream bottom anyways. Sediment comes the sides of the stream where NO dredging is allowed anyways.