How does placer mining use water to recover gold?

Placer mining is an ancient technique that utilizes the high density of gold to recover it from alluvial deposits. Because gold is significantly denser than the lighter siliceous materials (sand and gravel) with which it is typically found, water is used as a medium to separate these materials based on their specific gravity.123

The process generally involves the following steps:

• Excavation and Transport: Water is used to move gold-bearing soil or gravel into a processing tool, such as a pan, a rocker, or a sluice box.24
• Concentration: As the mixture of water and material flows, the current is manipulated to wash away the lighter waste materials. In a sluice box, for example, riffles (transverse bars) create eddies in the water flow. These eddies retard the current, allowing the heavy gold particles to settle and become trapped in the basins created by the riffles, while the lighter material is carried away.24
• Recovery: Once the lighter material has been washed away, the concentrated gold remains behind to be collected.2