I would not think a permit would be needed if your are not discharging back onto the bank or into the river. A permit is needed for a conventional highbanker. You might try asking D&K detectors in Portland or the mining shop in Sweethome.
Todd
I wouldn't think a recycling sluice box would be a problem either.
Erosion would not occur.
GPAA.com would be a good source for info.
Also get you onto private sites they own.