Rick, It will vary, but as an intelligent guess say probably no more than a mile. Gold is heavy and even very fine flakes will sink fast. This is all predicated on water flow and stream obstructions. My club has a favorite placer that always has gold in it. There is no "mother load" but because of the nature of the geology in this particular stream you will find fine gold in its entire length. Hope this helps.

Chauncey Hood, Idaho Gold Prospectors Association, Pres.