Grothe states that the first dredge was operated on the Clutha River in New Zealand in 1864, but even in 1891 the method had made but little progress, and was generally looked on with disfavor everywhere except in New Zealand. This method of recovering gold from the gravel of river beds has of late years made remarkable progress, and its extension to the working of all flat placers has completely changed the aspect of shallow alluvial mining. The gravel won by either of these three methods is washed in the usual way by sluicing.

Pits are sunk on the bank, and the gravels below water level excavated and raised to the surface by ordinary mining methods, or by the hydraulic elevator.

The gravel is raised by dredgers or similar contrivances, operated from a boat.

A portion of the river bed is laid dry by damming and fluming.

River mining consists in the working of auriferous gravels in the channels and beds of existing rivers, and may with convenience be made to include the exploitation of deep bars below the level of the water. The whole capital invested was often lost, and all works and machinery swept away by a flood before the pay-dirt was sighted, while numerous instances are on record in which the alluvium on the river-bed, after having been laid bare at great expense, was not rich enough to pay for sluicing. River mining was probably subject to more uncertainty than any other branch of gold mining. The operations were usually terminated by the autumn floods. The river bed exposed by such methods was prospected and the pay-dirt when found taken out and washed, particular attention being paid to the surface of the bed-rock.

Sometimes wing-dams were built out from the bank above and below the part of the river it was desired to work, and a third dam connecting their mid-stream ends was constructed parallel to the direction of the current.

Sometimes, tunnels were made to drain permanently large reaches and deliver the water at a lower point. This was usually done by building two dams from bank to bank, with their foundations on bedrock, the water being carried off in a wooden flume, starting above the head-dam and terminating below the foot-dam. In river mining, an entire river was frequently deflected from its course so as to lay bare a section of its bed. Gold dredging in now practised on the rivers of California, has is now superseded river mining.

To understand How Does a Gold Dredge Work, we may want to start by looking at California or Alaska.

Gold dredge pumps