(California's North Yuba River under normal flow conditions.)
There's always quite a bit of discussion among small-scale placer miners about what sort of water flow is necessary to move gold around in a running stream. Once again, I make a distinction here between wet and dry placers because the latter represent a unique entity for the most part. So the core of this post will be dedicated to water flow and gold movement in wet placers.
Level 1 Flow Conditions
I'm breaking water flow conditions down into various levels here, with Level 1 constituting what I'd typically call "normal" stream water flow. Now don't expect miracles in terms of gold movement under normal water flow conditions. Why? Simply because not much placer gold gets moved around with a steady flow that has little power behind it. I'll even take this a step further and state that virtually no gold gets moved around under normal water flow. Of course, Ma Nature can be a strange lady at times and anything can happen (and usually does). When I say little or no gold gets moved around I'm talking about even the smallest fines...forget flakes, coarser pieces, and nuggets. Most placer gold in a steady or normal stream flow will remain resting in place wherever it was deposited previously, snug as a bug in a rug. Of course stream configuration can upset this apple cart at times but not much. You see, it takes a lot of force and power to move gold around in a wet placer, including those tiny microdots or fines. This is simply the law of physics at work. Remember, even the tiniest pieces of gold can be very dense and heavy. Despite this fact some small-scale guys and gals continue to believe that gold is constantly moving under normal stream conditions. Well, they're entitled to their beliefs. But the law of physics as it applies to gold deposition doesn't lie. For the most part gold deposition follows physical laws, not supposition, wishful thinking, or tall tales passed around a campfire.
(Even the turbulence created by these North Yuba River rapids under Level 1 Flow will not move gold around.)
Level 2 Flow Conditions
Under Level 2 water flow conditions small-to-large rocks will be lifted up by the current and start tumbling their way downstream. By large rocks I mean those with a diameter of about six inches or so, or slightly bigger than that. In this sort of flow fine gold will begin to move and be carried downstream as well as some very small or thin flakes. Coarser gold, "chunkers," and nuggets will typically remain in place under Level 2 conditions. It takes a lot more hydraulic power to move the bigger stuff and under Level 2 conditions this isn't going to happen. The finer gold will be scoured out or lifted from its current resting places because fines tend to be buried at more shallow depths than heavier pieces of gold. Remember, I said typically. That's my CYA statement for those of you who have recovered lots of fines deep down or in bedrock cracks or crevices. Usually though, the lighter stuff is closer to the stream surface and when those fist-sized rocks get to moving around under Level 2 many of these fines are freed and make their way merrily downstream.
(Level 2-3 flow conditions.)
Level 3 Flow Conditions
These are essentially your generic flood conditions and are usually marked by extremely fast currents and high water conditions. Level 3 conditions are strong enough to get large boulders rolling along and create a powerful "soup" of debris, sand, gravel, and big rocks to accompany those big boulders. These sorts of floods can last only a few hours or, in more extreme circumstances, for days. They can result in flooding in lowland areas, but usually not to a severe degree. Level 3 is where the heavier pieces of gold start moving around, with nuggets and larger pieces tumbling end over end near the bottom of the stream bed. Where they come to rest is anyone's guess because of the variables involved. These include stream configuration, flow force, flood strength and length (time-wise), and what obstacles still remain in the path of those heavier pieces and nuggets. Under Level 3 conditions heavier gold packed tightly into bedrock cracks and crevices will not move but instead remains in place for the most part. In fact, gold already in those cracks and crevices may be joined by distant relatives carried downstream to their new resting location (and a happy reunion with family members!).
Level 4 Flow Conditions
It's "Maggie grab your drawers and haul ass" time under Level 4 flow conditions. Level 4s are your 100-year type floods and are extremely frightening and damaging. I've seen a Level 4 in California's Northern Motherlode and it was a jaw-dropper in its collective power. Significant flooding usually occurs in low-laying areas and houses, cars, roads, bridges, and other infrastructure can take a beating. Under these conditions huge boulders can be tossed around like beach balls and nuggets and coarse gold can be churned up from bedrock and lifted and carried some distance. Entire new placer gold pockets and paystreaks can be created in locations or at depths that essentially go against the norm in terms of deposition physics. It's anything goes under Level 4 flow conditions. In fact, the term flow doesn't even apply here. We're talking raging flood conditions. The extreme mixing of boulders, rocks, and gravels in Level 4s mean that a lot of the heavier gold is left closer to the surface when Level 4s end (their time frame is usually counted in days instead of hours). That's why you've heard me tell you before that you need to hit those gold-bearing areas that have undergone this sort of savage transformation as soon as possible after the event has passed and safe conditions exist. Extraordinary gold finds can be made both in size and quantity if you know what to look for and know what you're doing. Trust me on this...I've been there and done it. Eventually, however, as time passes ever so slowly and stream conditions return to normal, that heavier gold will start sinking ever downward and will eventually make it to bedrock.
(Level 4 flow conditions.)
(Level 4 flooding downstream in Yuba County, California.)
Since I mentioned this factor a number of times already I think it would be beneficial to touch on it again. Streams that have steeper gradients are more readily affected by flow conditions as are streams with little in the way of twists and turns. These are what I call "chute" type streams and under normal flow conditions the gold they contain tends to move around more than more complex streams with less of a downhill gradient. In fact, under Level 3 or 4 flow conditions a good part of the gold these streams contain may end up far downstream and resting toward the MIDDLE of the stream bed. Bear this in mind when you're considering the various factors I've laid out in this post. It may help you to discern what's going on gold-wise in a given stream and how the various levels of flow might affect that stream.
That's all she wrote...for now at least.
(c) Jim Rocha 2016
Questions? E-mail me at email@example.com