(Keep an eye out for heavies like these old square-headed nails.)
There are a number of factors associated with how heavies move and drop in a stream or wash:
EROSION: This should be easy to grasp for one and all. Erosion is essentially the detachment of material away from the sides of a given stream course. What causes this detachment? All sorts of things, including land or mud slides, heating and cooling, or the force of strong currents dislodging rocks, sand, dirt, and gravel from stream sides or banks. Notice here that I spoke of potential gold-bearing material detaching away from the sides of a stream or wash and not from its bottom. The reason for this is that over 95% of a stream's energy is applied to the sides in an attempt to overcome the frictional resistance imposed by the stream course and its structure. Only five percent of a stream's hydraulic action is used for vertical cutting (up and down) or movement.
(Stream erosion on the outside bend.)
Solution Load: The gold-bearing materials being cut away or pulled into a stream course are called "the solution load" by engineers and scientists. The solution load can contain materials of all shapes and sizes, but the weights and densities of those materials (man-made or geological) are another matter altogehter. This is an important factor to understand when it comes to where heavies tend to drop out, so listen up.
TIP: In the solution load any materials that are too heavy to be suspended in the water flow tend to roll, scoot, or move intermittently along the streambed. This jerky movement of heavies tends to abrade or "erode" the bottom of the present stream channel and sometimes its sides as well.
TRANSPORTATION: The basic concept here again is quite easy to comprehend. Transportation is the movement or carrying along of solution load materials downstream via stream hydraulics (direction, depth, strength of flow, etc.). Once the solution load (i.e., material scoured from the stream sides or banks) is being carried merrily along in a wash or stream it gets a new name...the stream load.
Stream Load: Now things start to get a bit complicated. The stream load is composed of the solution load, the stream load, and a little something called the suspension load. The suspension load are those materials carried in suspension in the water flow and tend to include lighter, less dense material like silt, sands, dirt, wood, and so on.
DEPOSITION: As stream or flow velocity decreases, so does the ability of a stream or wash to move loads through it.
TIP: The heaviest particles or pieces of the stream and bed loads drop or fall first. The more abrupt the decrease of water flow, the more abrupt or sudden the drop of the heavies. They'll remain in place where they drop which is a function of the bed's configuration and the obstacles near it or on it. Only the strongest of currents will disturb these heavies from their current resting places since only a tiny fraction of stream flow energy is directed vertically in a stream or wash.
(Part of the bed load.)
Now you know why the old timers in the California Gold Rush went to the expense and great labor of diverting water flow around or away from gold bearing streams and rivers in the Motherlode (and in other rushes, for that matter).
This should get some of you thinking. Perhaps now you also understand why so many small-scale mining veterans (myself included) use visualization techniques to get a grasp on where that bed load is going, where the mid-range materials are being carried, and where the light stuff is. It's a visual puzzle based on hydrology that, if solved correctly, can lead you to lots of yellow metal without moving tons of earth.
There's more to come...
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at email@example.com