Gravity Dredging for Placer Gold


I am sure that this idea is nothing new under the sun, but I found very little information on this particular style of gravity dredge when I had searched the internet. I assume the reason for this is that the tools that are used total to less than $20.00 and can be easily bought at your local hardware store. Which means that there is no real way for somebody to sell you anything more in order for you to operate a gravity dredge. Actually, that's not entirely true, I guess a wetsuit would be nice.

What is a gravity dredge? Well the type that I will be explaining is simply a piece of Big-O tubing. If you have no idea what it is, then take a drive to your local Home Depot store and ask to see their selection of non-perforated(solid) 4 inch Big-O piping. It always comes in one color, black. And you will notice that it is a flexible plastic pipe with riffles along it's entire length. This tubing is really a long sluice box, and it only costs about 50 cents per linear foot.

There are about 18 riffles per linear foot of pipe, so a 40 foot length of pipe would have over 700 riffles in order to catch gold, and the cost of a 40 foot length of pipe would be $20.00. There really isn't anything else necessary, except an explanation on how to make this pipe work for you, which is really the point of the rest of this article.

If you have ever syphoned gas from a car or water from a pool, you'll have used the same principle as this gravity dredge runs on. This dredge needs a drop of about one foot for every ten feet of length in order to work properly, if the drop is not enough, the riffles will plug up just like in a sluice box, and if the drop is too severe, then everything will be washed through the pipe. But a good general rule is to place a variety of large rocks through your gravity dredge when it is running to make sure that they all do come out the other side, but just barely. The gold and black sands will drop into the riffles and stay there as the bigger rocks tumble on through.

First of all, you should find a suitable location that has the recommended drop rate, and I would suggest that you look for a pool on the stream that you want to dredge with a 2 or 3 foot drop on the downstream exit from this spot. It can be a waterfall or just a steep grade, but as long as you have some drop in the stream that can create the effect needed for this gravity dredge.

The next thing that you want to do is measure the length of pipe that you will need. Big-O piping can be purchased in any length up to 100 feet, and I would suggest that you not try to use more than a single piece, only because it is critical that there is no air leaks in the entire length of pipe, and a connection point would be a weak spot for air leaks.

Once you have your pipe and a spot picked out, the next thing to do is tie your dredge down, so that the weight of the water won't cause the dredge to wander down stream. The best way to do this is to get a piece of rope and tie it around the pipe at some spot just above the grade drop, and then tie the rope to a big rock or tree nearby. Now attach a 2 pound rock to each end of the dredge using duct tape, in order to hold them under the water. This is because gravity dredges hate one thing: air. So if too much air is sucked in through the intake, it will stop working, or if the outlet end of the dredge is allowed to come out of the water at the lower end, it could draw air in through that way as well. Actually you do not have to put the exit end of the dredge underwater if you create an upturn at the end of the pipe in order to create a trap so that air will not flow back up the pipe.

In order to make your dredge work, just like the gas syphon, you must fill it with water. The easiest way to do this is to plug the lower end of the pipe with an end cap(also bought at a hardware store) and place the intake end only halfway under the surface of the water to allow air to escape as the water flows down into the pipe, filling it up. When the pipe is full, then allow the intake to drop to the bottom of the pool, due to the weight of the rock that you duct taped to it. Then remove the end cap from the lower end of the pipe. If you watch the lower end of the pipe, you should see air bubbles leaving with the rush of water. Make sure that all the air is flushed through before you start to suck up gravels.

If you left too much air in the pipe, it will only run at half speed, this is no good and you will be struggling to avoid plug ups. A way to test this out is to try and pick up the pipe at a spot that you suspect might have air, usually a high point, if it is too light to be filled with water, then you must try to fill it again. Sometimes you can clear the air by plugging the bottom end with your hand and waiting for 20 seconds for the water to backup, and then quickly releasing it. If you see air leaving the pipe, then try it once more after the bubbles stop coming out. A good sign that you have cleared everything out is that you can't hold the water back with your hand. This is really the hardest part of the whole gravity dredging operation, so don't be discouraged if it takes a few attempts.

Now that the water is rushing through your Gravity Dredge, we now have to test it to see if the angle will allow the bigger rocks to pass through. So find some rocks that you would consider just large enough to put down the pipe. I usually try 3 inch sized rocks because the odds are that you will let a few of them down anyways. If the rocks do not come out of the pipe, you could have a high spot that the rocks can't quite climb over, and you can fix this by digging out the ground or lowering the exit of the pipe in order to create more water speed. If the rocks are exiting too fast, you may need to raise the exit of the pipe enough in order to have the big rocks just barely fall out of the pipe.

Depending on how your pipe is laid out, there will be sections in your pipe that will catch black sands and gold, and there will be sections that won't. If your pipe uniformly drops down to it's exit point, it will be the best case scenario for catching gold. But if you are in a place that has a waterfall type grade, the part of your pipe that is steep will not catch gold, but the long flat length leading up to the drop will catch the gold. Either case will do well as long as you do not disturb the length of pipe that has the gold and blacksands too much.

When you are dredging, it is recommended that you hold the intake so that your hand is partially in the pipe, thus making the intake the smallest diameter of the length of pipe. If you should not notice a 4 inch rock and accidentally suck it up, it will stop up against your hand, and you can easily remove it, and prevent a plug up. If you find this method uses up too much of your resources(mainly one of your hands) then you should buy an insert for your pipe, it is really used for joining pipes together but we want to use it for making the intake about half an inch smaller in order to stop any rocks 3 1/2 inches or bigger from going into your 4 inch dredge.

You will notice that it is really quite difficult to see what's happening at the intake of your dredge, unless you have a mask on, or you are using a water window. A water window is a piece of window glass that you silicon a wooden frame around so that it floats on the water and allows you to see clearly into the stream. I would recommend that you build your window just big enough to fit into a 5 gallon bucket, so that it's easy to carry.

A Gravity Dredge can move a lot of material quickly, but that's only one aspect of it, the most appealing part of dredging is the simple fact that you can clean the bottom surface of a bedrock stream bed to the point where there is absolutely nothing left on it. And cracks are easier to clean up by simply placing a dredge overtop of it and vacuuming. There might be the odd rock that needs encouraging with a prybar or a screwdriver, but a dredge is the best bedrock cleaner you'll ever find. Any size less than 4 inches will force you to throw out too many stones from the gravels that you are dredging up, and if the dredge is bigger than 4 inches, then you will find it too hard to maneuver into the various places.

In order to clean up your dredge, you should place your gold pan over the intake to slow down the water flow. Then place the exit end into your 5 gallon bucket with lots of big rocks inside the bucket to hold the pipe in place. Then gradually introduce air into the intake by raising it out of the water while holding your gold pan up against it. Once the gravity dredge has stopped syphoning, the next step is to start at the intake end of your dredge and hold it up vertically while hitting the sides with your hand or a rock to dislodge the black sands and gold from the riffles in the dredge, this is really like cleaning out your sluice and you want to hit the pipe untill there is no rattling of sands to be heard in that section, then you can proceed to the next lower section. Continue this process for the whole length of the pipe until you arrive at the bucket with the pipe in it. Once you have shaken all of the contents out of the pipe into the 5 gallon bucket, you can remove the big rocks and then empty the rest of the contents from the bucket into your gold pan and find out what you have dredged up for the day.


General thoughts

I tend to try and carry everything into the site in one run, so I usually don't bring a prybar or a cum-a-long for the first day that I dredge out an area, and I will wrap the big-O up with the bungie straps to keep it in a nice circle. Everything else should fit into the 5 gallon bucket. If you don't have enough grade, it is possible to make a small dam on a small creek in order to raise the water level of the spot that you are interested in, thus creating the necessary drop. As with every sport, there are a few words of caution. First, be careful about dredging under rocks that will eventually roll after you have removed enough material from under them. This is the worst danger, and it could pin you under water. The other danger is that you could get too cold if you are so excited that you are not aware of your body's temperature. This is really something that should be done with caution, and it is easier with a friend. Because of the safety issue, but also, one guy could be moving the bigger rocks that take two hands, while the guy working the dredge vacuums up the gravels and tosses the loose rocks that can be tossed with a single hand. It's very easy to see that two guys can actually do more than twice the work of one guy if the job is done this way because it saves the time of having to put the dredge down to move large rocks and then picking it up again. Have a fun and safe prospecting trip. . . . . . . This site is a little plain mainly because I spent less than a day figuring out html (the web site platform language used here) and I wanted to build something sooner than later. Anyhow my Email . . . . rlnwat@telus.net if you have thoughts and ideas that we can mesh together to improve these concepts. Thanks, ron.


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After thoughts: The style of riffles in these big-O pipes are actually different from one supplier to the next, so you might have to experiment a little with the grade. The type that I was using was from Home Depot and it tended to not have as deep a riffle as others that I have seen, but that also meant that I was able to rattle the contents out of it easier than a sharp-angled riffled pipe. But they both have their merits and either will catch gold. The gold size that I was catching was down to about 1/100th of a grain and up to 7 grains in size. I did also place two sluices side by side at the exit of the pipe to see if I was missing anything, and there is about a 20% loss with this process, but gold size would make a difference, bigger gold works better and I'm sure a half ounce nugget wouldn't slip through the system.

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