In California a trommel is considered a classifier and is unregulated. You can use it year around and about the only restriction you have will be discharge of muddy water back to the stream or digging in the wrong upland areas. Now if you start moving 100's of yards of gravel I imagine you will have to post a bond and file and reclamation plan etc etc... but anything you do with a small portable trommel is completely allowed.
I have a Gold Screw trommel and it's one of the finest mining machines ever produced. It has a reverse helix spiral to pull out the concentrates and a built in gold wheel to finish the cons. Everything runs off one motor including the pump and there is no further cleanup. You shovel into one end and gold falls out the other in about one minute, completely automatic.
You can even feed it with a small excavator with the proper grizzly attached.
Email me if you want to buy it, I would rather dredge and I have way to many toys. bonaro123@yahoo.com