Yes, they mined for gold with the bucket dredges many years ago and sluiced out the gold, or most of it anyway. But you never get it all. It’s a question if it is economically recoverable.
See how they do it on Discovery Channel’s “Gold Rush.”
The killer for the Hoffman's backup operation in Oregon was they didn't count on the old dredge miners capturing all of the worthwhile gold (deep) at the bedrock boundary. Dave Turin & Todd's assumption was that the ol' timers dredge buckets weren't able to cut into the top cracked seam layer of the bedrock - as could modern excavators. Turned out to be a bad assumption.
It's an interesting show. The behind the scenes "extras" from a few years ago revealed the introduction of using high tech drones. They had some safety concerns about buzz cutting the miners in close up shots (stopping & hovering). I'm more interested in the show due to the character dynamics & management styles. There is a wealth of information for people to learn who put teams together & have to make tough decisions with different character types.
If you are ever driving by Yerka, CA on I-5 stop and visit the county building. There are images of the Klamath River being dredged by ever larger dredges. As far as I know it was dredged at least three times from the ocean to almost Klamath Falls. The same happened to the Feather. BTW when the old dredges hit pockets of clay it would remove most/all of the gold from the sluices. A good operator would recognize it and bypass the sluice box until the clay streak was past. However, they dredged at night and it was often hard to tell what the buckets were bringing up in the feeble lights available at the time. Point is that there is still spots of gold, even after a dredge passed over an area, they are just tied up in clay.
Don’t get me started on Todd Hoffman. I can do a lengthy diatribe on what a putz he is. Beet’s is reckless and Parker can become a great miner if he keeps growing - he wants to be like John and John was an infinitely patient man.