With lots of rain, comes lots of runoff into the streams and rivers. That runoff often brings with it lots of material that's been buried for hundreds (or more) of years. Always new material getting added to the streams & rivers that might make someone rich one day. Or not; YMMV.

I don't know about dredging, but panning for gold or other "valuable" materials can be a fun activity. While I had fun when I did it 20 years ago, I never really expected to get more than just some conversation pieces out of it.