Gravity has long been the prospector’s friend.
Gold is found in riverbeds simply because the flow of water, over time, erodes mountains and carries minerals away in the form of flakes. Gold is found in riverbeds by prospectors because it is heavy, so the gold flakes will sink to the bottom of any pan in which river or stream water and sediment are mixed and swirled about.
Gravity as an Ally
Gold is heavier than silver.
Specific gravity is the weight ratio of a material compared to the weight of the same volume of water. Specific gravity is temperature dependent, but given the density of water at 4 degrees Celsius, the specific gravity of gold is 19.3.
For purposes of comparison: The specific gravity of silver at the same temperature is 10.5; copper is 8.9. Gold, in short, is heavy, and this fact means that gravity has always been a key ally in finding it.
Why It Is in the Stream
There’s gold in them thar’ lava streams!
A look at the geology behind the well-known gold-strike at Sutter’s Mill in 1848, the catalyst of the “49-ers Gold Rush,” gives some insight into why the gold is in the stream in the first place. An understanding of the geology of the location of gold around the planet’s crust has to include examining volcanoes, plate tectonics and erosion. Volcanic activity can lift gold from deep inside the earth onto the surface as one of the elements in the lava flow. Scientists believe that between 400 million and 200 million years ago, a tectonic plate constituting sea floor, with volcanic islands and gold deposits, was pressing eastward. Meanwhile, the North American continent and its tectonic plate were moving westward.
The Sierra Nevadas are a source of gold.
The collision of the two plates created what we know as California, and created the mountains with their rich gold deposits. Of particular historical importance are the Sierra Nevada goldfields in the north and east of that state. The Feather and the American rivers both run through these gold-bearing mountains.
From that point forward the geological part of the story is fairly simple. Flowing water is, over time, a very powerful force and can decompose rocks, all the while suspending within the water (if it is running with sufficient speed to prevent settling) the mineral grains or flakes formed by this decomposition.
The Chemistry of Erosion
The erosion caused by flowing water has a chemical, as well as a mechanical, element. Where the water is acidic, chemical reactions will separate the minerals suspended in the stream into their constituent elements.
A lot of things, then, may have happened to put the gold in the river.