To Dredge for Gold you need a Gold Dredge.
This is a machine on pontoons that has a water pump and sluice box on it.
A suction hose is run from the gold dredge to the riverbed where the operator manoeuvres the nozzle to suck up gold bearing gravel.
Folks who have never seen a gold dredge often assume that gravel is sucked into the water pump then discharged over the sluice. This is not the case.
The water pump sucks fresh river water which is then injected into the dredge hose via a Suction Nozzle or Power Jet at reasonable force. This action creates a venturi effect that creates the required suction.