If anyone is interested in some information on the potential impacts (both positive and negative) from recreational gold mining you may find the following interesting -

http://wdfw.wa.gov/publications/00293/wdfw00293.pdf

On the whole a balanced work with lots of citations from a number of studies supporting the information provided. Those citations would be a good place if anyone wants to look into more detail information. Section 7 provides a nice summary on some of the issues discussed earlier in this thread.

Curt