How Sluice Boxes Work
Running a sluice box is one of my favorite methods of prospecting. With relatively little equipment to pack in, a prospector can move a lot of material and develop a good amount of concentrates in a day. Sluices also have the advantage of being hand fed, non-powered equipment, so no gas to pack, no smoke to inhale and no noise to spoil a quiet afternoon on the stream, plus the added advantage of far fewer regulations. Powered equipment is fun too, but nothing beats a sluice box for production in lightweight, hand-fed equipment.
To understand how sluice boxes work, you first want to think about how gold deposits in the river. We all know gold is heavy - really heavy. A single gold bar in Fort Knox weighs about 42 lbs, but if you're just starting out you might not consider that, in order for gold to concentrate where it does in a stream, it must first be carried by water.
Most mountain streams undergo major transformations during the spring thaws, and times of heavy rains. Many go from peaceful little creeks to raging torrential rivers.
At one spot on the Rogue River Trail in Southern Oregon, you hike past the high water mark from the famous 1964 flood. The mark is about head height and the river is...way down there?!?! The Rogue River reached 500,000 cubic feet per second at Agnes during the '64 flood. To put that in perspective, the normal flow of the Mississippi River at New Orleans is 600,000 cubic feet per second. The Rogue River in flood nearly matched the mighty Mississippi for water volume!!! All that water driving down a narrow canyon moves pretty much everything it comes across, gold included.
So the force of flood water is enough to pick up and carry not only silt, sand and gravels, but also gold, large rocks and even boulders. Stand by a river in flood stage and you can often hear the larger rocks and boulders bouncing down stream and cracking into one another. This material is carried downstream in suspension. That means the material is suspended in, and flowing with the water. Gold settles wherever the water slows enough to allow it to drop out of suspension.
Rivers aren't like plumbing pipe, with smooth walls to guide the water at a consistent speed. Rivers flow through channels with irregularities, rock outcroppings, bends, narrow spots, wide spots, intrusions of bedrock, etc. Even the Los Angeles River, in its concrete channel, has seems in the concrete, bridge footings, bends, shopping carts and other debris, side channels joining the flow, etc. These irregularities mean the water flows at different speeds at different points. Anywhere the water slows, material will drop out of suspension and that's where the gold will be.
Sluice boxes work by essentially creating a straight, consistent channel, with regularly spaced slow spots created by riffles. Each riffle creates an eddy, a backflow of water that allows the gold to settle out. Material is placed at the top of the box and carried in suspension down the channel. The gold drops out of suspension as the water slows on the back side of the riffles.
Sluice Box Set-Up & Use
Getting the flow right is the key to running a sluice. Too much water, moving too quickly will carry gold higher in suspension. The science might be tricky to explain, but basically the upper layer of water is not affected by the riffles as much as the lower layer. While the lower layer is rolling behind the riffle, the upper is flowing right over the eddy, and while the eddy itself can slow the upper layer some, it's not as effective as the riffle itself.
On the flip side, too little flow, while allowing the gold to drop out quickly, also allows lighter material to drop out, filling the space between your riffles, eliminating the slow spots entirely, and allowing the rest of your gold to flow right out of the box.
The perfect flow allows the gold and other heavies to drop out of suspension and the lighter material to flow out of the box.
The general rule of thumb is 1" drop per foot of length of your box. So if your sluice box is 24" long, you'd start out with about 2" overall drop from upstream to downstream ends of your sluice and adjust from there.
Now finding a spot in the stream that allows the flow and drop required to get your box running just right isn't always easy, but it's better to set up in a good spot to get the right flow and carry your paying material to your sluice than it is to set up in a bad spot close to your diggings.
In some areas it may be impossible to find a safe spot with the right flow and drop, and nobody wants to stumble and drown while lugging buckets to a sluice perched over the swirling abyss. In these areas you may be able to increase flow of a slower, shallower and safer section of stream by setting up a wing dam to hold your sluice and manage the flow. A wing dam is essentially a rock barrier, often extending in a V upstream and outward from the mouth of your sluice box, that channels water into the box increasing flow.
Depth of water flowing through the box is often dependent on circumstances, but you do want the water flowing through the channel of the box, not over the top of it. I usually shoot for a water line about midway up the side of the sluice, but don't agonize over depth. It's flow that matters most.
Once you get what you hope to be the right flow, it's always a good idea to plonk a large rock across your box to hold it in place. The risk of losing your box or even just having it shift and dump your day's work is enough to warrant spending a minute to find a rock for the job.
Now you dig. Hopefully you've sample panned the area and found some decent ground before you set up. If not, you probably want to do that before you start digging. Gold does not settle out evenly and is not distributed evenly throughout the gravels of a stream, so it's always a good idea to pan material from a variety of likely spots and pick the best one before you start doing the heavy lifting.
At this point, you can classify the material to pull out the larger rocks, or run as is and pick them out by hand or sweep them with the current out the end of your box. The choice comes down to a matter of preference and the size of gold you're after. Classification adds time, and means running less material overall, but it's always easier to separate gold from material of a similar size, and larger rocks in a sluice can create unpredictable currents that may dislodge your gold and send it out the end of the sluice box. Nuggets aren't as likely to wash out, but if you're digging an area rich in fines, large rocks can be a problem. Personally, I like to classify my material before I run it.
Feed the material into the top of your sluice slowly, and give it plenty of time to wash through the box before adding more. Overloading will only clog your riffles and send gold downstream. I use a large garden trowel and run one scoop at a time.
When you're ready to clean up, and when is another judgment call you'll have to make based on the material you're running and experience, let the water flow for a bit after your last scoop. Then carefully remove the rock and lift the sluice box out of the stream. Be careful not to slosh water and material out of the box. Place the downstream end in your clean-up tub or bucket and inspect for visible nuggets. Grab any nuggets or pickers and put in your snuffer or other container for safekeeping. Then release the riffle tray catches. Rinse the riffles before removing or folding out of the way. Slide out the carpet, miner's moss and/or matting and rinse well before setting aside. Rinse everything into your tub or bucket.
Now it's entirely up to you if you'd like to pan down your concentrates or get your sluice back in the water and do some more digging. I'll usually pan a quick sample into my tub just to get an idea of how I'm doing, but I can always pan out my concentrates at home, so digging and sluicing more material is really the best use of my time on the stream.
There you have it. Pack out your trash, fill in your holes and best of luck sluicing for gold!
If you are interested in buying a sluice, we sell a wide variety in our Sluice Boxes.