Why does gold settle out of moving water during placer mining?

In placer mining, gold settles out of moving water primarily due to its high density (19.3 grams per cubic centimetre) relative to the lighter siliceous materials with which it is typically found.12

When water flows, it acts as a transporting agent for mineral grains of varying densities. Because gold is significantly denser than the surrounding sand and gravel, it sinks more rapidly as the current slows down. This process, known as gravity concentration, allows miners to separate the heavy gold from lighter waste matter. Techniques such as panning, sluicing, and the use of riffles or baffles are designed to exploit this physical property; they create areas of reduced current or small basins where the water eddies, causing the heavy gold to settle and be trapped while the lighter material is washed away.31456