A gold dredge is a simple placer mining machine used to extract gold from gravel, dirt and sand using water and
mechanical methods. Placer mining is the technique of mining for minerals in alluvial deposits, done through open-pit or by various surface excavating equipment or tunneling equipment.
Since the beginning of the Gold Rush, the basic concept of placer mining has not changed. The concept is that
gold being the heavy/dense material in the sand or soil will always settle to the bottom, leaving the sand, rock
and dirt, which will wash away. Gold panning, sluice boxes and rockers were the original methods used for
placer mining. All these methods involved washing sand, dirt and gravel in water, leaving behind gold that settles
to the bottom of the pan or into the bottom of the riffles of the sluice box.
In the first half of the 1990s, the original gold dredges built were large, multi-story machines. These steam-
powered dredges were used along the rivers in California. As the easily accessible gold in California was soon
gone by the mid to late 1850s, there was still a lot of gold there to be mined. It took a professional approach,
giant machines and giant companies, to retrieve that gold. In the early 1990s, massive floating dredges scooped
up millions of tons of river gravel, as steam power and electrical power became available.
From the 1990s to date, dredging has become as a popular way of gold mining. Advancement in technology has
allowed a small dredge to be carried by a single person to a remote location to profitably process gravel banks
on streams that previously were inaccessible to the multi-story hundreds-of-feet-long dredges of the 1930s.
Dredges today are versatile, consisting of both submersible dredges and floating surface dredges that use
vacuums to suck up gravel from the bottom.
There are different types of dredges, the most popular of which are:
The Surface Dredge: it floats on the surface of the river or lake. Through a suction pump, the silt and other
material is pumped to the surface through a suction hose to a sluice box. The sluice box processes all dredge
material into a single recovery box. It uses a short classifier screen at the entrance to separate the larger
cobbles from the smaller ones, which are most likely to contain small gold particles. The sluice box either
floats on the surface of the water or rests on a bank.
Modern sluice boxes come with multi-stage systems, the most advanced is called the 3-stage sluice box.
It is needed as it can recover the finer grade gold from the material pumped with the dredge.
The Underwater Dredge: this type of dredge is less popular as it is not as efficient in recovering the finer
grade gold. It is cumbersome to handle underwater and cannot reach hard to get areas on the riverbank.
It has a flared metal or plastic tube with an attached metal elbow at a forty-five to sixty-degree bend.
High pressure water is pumped into the bend, creating a vacuum at the end of the bend. A riffle tray is
attached at the end of the tube containing a series of gold traps. As the gravel is sucked into the dredge,
the heavier particles, including gold, become entrapped in the riffle tray. The lighter, non gold bearing
particles flow back into the river.