Interesting questions. Some of the gullies i visit are well frequented or new to me, and so its always worth trying your luck, i pulled a really great haul crevicing downstream of one gully where it had clearly been creviced upstream, i was actually doing a 101 camera shoot to add in here (for education and critiquing) when the good stuff came out. If i put it up well someone would work it out and beat me to any other good ones in the area. That crevice held a particular type of gold that made me think it would fill again in time, but its hard to prove, I can only surmise.
Which leads to flood gold deposits. With enough swell and turbulence a water course can very quickly move some of the lighter free gold around redepositing in good areas within the gold line. I recovered 80 odd specks of flood gold after that three inches of rain (because I'd spent the past 6 hours for zero test panning a few barren gullies in the same area and needed to restore my faith) that were definitely new deposits. I can say this conclusively because i highbanked that same lip of a deep hole to the bedrock only two weeks beforehand. I was methodical and the gravel deposits were "new" arrivals due to the midsize flow since my last visit.
Try looking at stream dynamics and apply these theories in your situation. Worst case scenario sampling is you get zero. I never think of an area as proved or barren by guessing, only by sampling. A massive flood where water courses surge and redefine the stream bed are a definite green light to get out and try your luck, it can yeild good results, but the serious will try and find the old deposits, or accumulated deposits, that's where reward for effort really puts some serious colors in the pan. Finding something that has taken that 30 years (or more maybe hundreds) to accumulate is always on the to do list.
