Original gold dredging began in the very early 1900’s, mainly to harvest gravels and sands for gold in previously worked placer gold fields. Some of these gold fields are the California ones of the Sierra Nevada 1849 gold rush, the Yukon one of the Klondike area near Dawson City and Atlin, BC.

These gold dredges typically harvested the sand and gravel from the bottom of the streams and rivers which had been inaccessible to the old-time prospectors who were limited to the river banks and peripheral sand bars. Prior to the advent of the dredge, placer techniques were mainly limited to gold panning, and use of rockers and sluice boxes, and later hydraulic monitors. These original dredges were in operation up until about 1966. After WWII, increased wages and the fixed price of gold eventually took their toll on profitability, causing dredging to cease.

The original gold dredges were mostly wooden and were quite large. The largest one in North America has been preserved near Dawson City in the Yukon. This is the No. 4, and it was 2/3 the size of a football field and eight stories high! At its heyday, it would move along in a pond made by itself, and its digging rate for gold-bearing gravel was 22 buckets per minute. It would operate for 24 hours per day (as do modern mines) for a season of approx. 200 days, April – November, but weather dependent. Over 46 years, it unearthed 9 tons of gold, grossing 8.6 million dollars for an average of $1000/day. In 1912, in the Klondike, there were over 13 dredges working. Dredging was mostly controlled by mining companies’ not individual prospectors as they were too expensive.

Gold Dredge Types -Working & Operating Principles

The original dredge operating procedure was as follows;

1. The bucket line on the bow dug up gold-bearing gravel and deposited it to the hopper
2. The hopper fed gravel into the trommel
3. The trommel was a revolving tube-like metal screen, constantly rotating
4. Water was sprayed through the trommel, washing off the gravel. Holes in the screen would allow the gold and fines to pass through to a sluice box
5. The gold would settle in the sluice boxes for collection
6. The waste gravel moved along a stacker belt
7. The resulting tailings piles would be deposited out the stern in a scalloped pattern.

As well as gold, the dredge recovered anything it came across, including a prehistoric mammoth ivory and a set of false teeth!

In recent years, dredging has become re-activated. While there are some new somewhat large dredges being built still, the most popular kind are small and lightweight which can weigh as little as 40 pounds and cost only around $800. In other words, up until the 1960’s, dredging was only affordable to full-fledged mining companies, now it is available on a small scale to the individual prospector, somewhat like rockers and sluice boxes were in the 1800’s.

As for these new small dredges, there are three main kinds, the surface dredge, the submersible dredge and the underwater submersible dredge. The surface dredge floats on the surface of the water. The sand and gravel is pumped through a suction hose into an efficient sluice box. This sluice box is capable of recovering fine particles of gold. The surface dredge can easily be operated with or without diving equipment. The underwater dredge is the less popular of the dredges available nowadays since it is less efficient at recovering as fine a gold as the surface type.

Regarding the general operation of these gold dredges, the dredge is operated by the use of a water pump and a network of suction hoses. The hose sizes can vary from 1 inch to 20 inches or more. Once vacuumed up, the dirt and gravel is discharged into a sluice. The sluice box contains horizontal ridges or bars, or a series of horizontal channels that act to catch the heaviest particles (the gold) in the water/dirt mixture as they are vibrated and washed down through the sluice box. After passing through the sluicebox, the slurry is discharged over the side of the dredge and directed to a settling pond. After a few hours of operation, the contents of the sluicebox are placed in a gold pan and the gold can then be collected. This, then is the way the modern prospector can finally utilize the principle of dredging for his small-scale operation.

http://outdoorchannel.com/article.aspx?id=16804&articletype=article