What is Dredging?
Dredging is the most efficient means of recreational prospecting. Dredging is the use of a suction dredge to "vacuum" gravels off the bottom of a creek.
Consider a dredge as a sluice box either mounted on a 4-legged stand or mounted on floats. the sluice box is held by the stand or floats at the proper angle, just like it is when sluicing or high banking. Gravels enter at the top of the sluice box. An engine pump combination is either mounted on the floats with the sluice box, or placed on the shore.
High pressure water from the pump travels through a hose to a "jet tube" or a "power jet nozzle." Either way, a suction is created in the suction hose because of the venturi principle. The suction developed is sufficient to suck-up gravels, rocks, and gold!
Dredges come in a variety of sizes from 1 1/2 inch backpack dredges, that can be carried easily to a remote location, up to 12 inches. The measure of the dredges size is the diameter of the suction hose - not the nozzle. But, nozzle opening diameter IS the measure that states use to limit dredges. Generally, recreational prospecting is considered to have an upper limit of 4-6 inches, depending on state law.
The suction nozzle, usually made of steel, is the diameter of the suction hose, but the opening of the nozzle is usually necked-down with a restriction ring welded on to decrease the opening by about a 1/2 inch. This decreases the possibility of a rock plugging the hose farther up, and also allows a larger dredge to meet state restrictions on nozzle sizes. As an example, a 3 inch dredge (hose size) with a nozzle opening of 2 1/2 inches can be used in an area that has a maximum dredge size restriction of 2 1/2 inches.
Differences in dredge design varies as much as other recreational prospecting equipment. The entrance of of the gravels into the sluice box will be by either a "Header Box" that slows down the water before entering the sluice box, or a "Jet Flare" that is a long flaring tube that slows the water down. The header box has worked for many, many years and works fine, but the new rage is the jet flare dredge. The reasoning is that the jet flare provides a little more suction at the nozzle.
Some smaller dredges use inner tubes as floats, where most dredges use plastic pontoons. Some pontoons are inflatable and can be collapsed for easy transport. Some dredges are high banker-dredge combinations, and have a 4-legged metal frame holding the sluice box. These are used only in shallow water. These same high banker dredge combinations can also be fitted with pontoons as an option.
The larger dredges (3 inches and up) can have a breathing air pump mounted on the engine-pump to provide compressed air to the dredger, who can dive underwater to get to gravels not accessible otherwise. This is where the real value of the dredge can be seen. Without compressed air, you can't get to these gravels, and neither could the old-timers (unless they diverted the entire river, which they sometimes did). The bottom of the river may have cracks in the bedrock that has trapped gold for a thousand years just waiting for the first dredger to come along and vacuum it into his sluice box.
The dredge is an extremely efficient piece of machinery. If you are just beginning your journey into recreational prospecting, I recommend getting a 2 1/2 to 3 inch dredge high banker combination. This rig is versatile and efficient, and can recover a lot of gold.
If your already into recreational prospecting, I recommend getting the largest dredge you can handle and afford, but look at the state laws you will have to work under. States like Washington limit recreational prospecting to 4 inches. As of this writing, California has put a "temporarty" moretoreum (since summer 2009) on suction dredging in the entire state. In other words, it does no good to have a 6 inch dredge when you can't use one larger than 4 inches...or not at all.
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