In Oregon, dredging is not allowed in Essential Salmon Habitat (ESH). Those waters can be found on the Oregon DEQ website on their interactive map. Dredging is still allowed in non-ESH waters, and the seasons for that can be found for the in-water work periods on the Division of State Lands site. Varies with the waterway.
Panning (no permit required) and sluicing allowed in both ESH and non-ESH waters all year. Dredge permits (700PM) are required by DEQ, and prepare for sticker shock-$500 total for the first year... for sluicing, DEQ also says you need to fill out the 700PM application and keep with you, but don't have to submit or pay a fee. I have yet to be checked.
The Quartzville Group is open to dredging from June 1st to mid Oct, and the two claims out fo Cottage Grove (Golden Cat and Paradise) are open from May to mid Nov.
Don't disturb vegetation above high water mark and you should be OK most anywhere.
Edit: If you want to do any beach prospecting, no motorized anywhere on Oregon beaches, not even battery/electric, even in a recirculating system.
Running a hi-banker is a little complicated right now, supposed to have the 600 permit, that they aren't isuing right now, have grandfathered existing permits that are out, and the distance from waters has changed somewhat in the last year or so with SB3 passage. Have to have some kind of settling pond or other way to not allow sedimented water back into stream. Don't hi-bank so not up on all the stuff involved with it.
ope this helps and points you in the right direction. If you ever get down to the Quartzville Group, and need someone to show you around, holler at me, I am part owner in two of those claims and our Chapter has adopted all three of them.