Tuesday, May 24, 2016

"The Heavies Drop Out First" (Part 5)


 ("Like I told you dude, it's like wayyyyy heavy.")

We'll keep moving right along here on this heavies issue, just as those same heavies bounce and tumble along the bed of a gold-bearing stream under high water flow. So stick around. You may learn something new.

Stream Capacity

One of the most important factors governing material transportation in a stream or wash is "stream capacity." This is the maximum load of material of all types that can be carried or moved in a stream for any given water flow amount. Obviously, the greater the velocity of the water flow the greater the frictional "drag" along the bottom of that stream or wash and the greater the likelihood that the heavies will start scraping and tumbling their way downstream. One aspect of stream capacity that's very important is turbulence. The greater the velocity and the turbulence in a gold-bearing stream or wash, the greater the likelihood that those heavies will be transported in some way, shape, form, or distance. By the way, streams or washes with a decent angle of descent tend to be the best gold or heavies depositors.


Hint, Hint

Although hydrologists place the bulk of the emphasis on a stream's ability to move heavy materials on the shoulders of velocity, I myself tend not to. I think turbulence is as big a factor (if not more so) in moving heavies downstream and even up toward the sides of a wash or stream. Direct experience causes me to make this statement but, as you know by now, without decent velocity in a stream or wash there's not going to be much in the way of turbulence either. But try looking at this the way I do. A stream or wash that has smoother contours, less obstructions, and a lazy movement downstream is not going to be the best candidate for moving and depositing heavies to any great extent. If that same stream, however, is roughly configured, has lots of varied sized obstructions, and tends to have decent velocity under normal water flow conditions, then when the shit hits the fan from a velocity standpoint the associated turbulence will be just as important a heavies movement and deposition factor as the flow speed. In other words, gold, iron, lead, garnets, and other heavies will start traveling pretty quickly. Remember, under optimum conditions the friction factor along a wash or streambed's bottom will also increase. This causes a "scouring" effect that can move even heavy nuggets of gold if the conditions are right. That's why you've heard me tell you more than once that it's the "early bird who gets the worm" (hint, hint) after heavy flooding or flash flooding occurs in known gold-bearing streams or washes.

Two Main Factors

Under high-velocity water flow and great turbulence, heavy materials get moved in essentially two ways:

1) Traction (a sort of rolling or scooting along the bottom type of movement that can shove or push other heavies along with it).

2) Saltation (this is a "bouncing" movement that often knocks other heavies loose and sends them on their way to bound and bounce, repeating the entire process.)


Traction and saltation are the two main factors governing the movement of heavies in a wash or stream. Little light bulbs should be going off in your head by now in terms of gaining a new understanding of gold deposition. If not, relax and think on what you've read thus far and things should start falling into place from a conceptual/visual standpoint.

Chances Are...

What happens to those lighter stream-borne materials during high-velocity water flow? They will be suspended in the water from lighter-to-heavier starting at the top of the stream. They get carried farther, onto the stream or wash banks, and often end up wherever they drop once high-velocity water flow and turbulence end. So when you're prowling the sides or banks of that stream or wash and start finding color in the form of fines or even small, light flakes don't break into your happy dance just yet. Chances are the heavies you're looking for aren't going to be found there. However, they may be a bit farther upstream and hiding in likely stopping points closer to the stream's center. Granted, I've seen Ma Nature throw a few curve balls my way in this regard over the years, but in general you should abide by this rule of thumb.

 (Remember that Ma Nature will throw you a deposition curve ball every now and then.)

We'll talk some about desert or dry placer heavies next time. Until then be good to yourself and to those around you.

(c) Jim Rocha (J.R.) 2016

Questions? E-mail me at jr872vt90@yahoo.com

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