Every grain here obeys Stokes' law: the viscous drag on a small sphere settling in water gives a settling (relaxation) rate k = 4.5·μ/(ρ·r²) and an equilibrium sink velocity v = (2/9)·(ρ−ρw)·g·r²/μ. Gold's specific gravity (19.3) against water is nearly 8× that of quartz sand (2.65), so a gold grain accelerates toward the bed far faster than a sand grain of the same size — the entire principle behind panning and sluicing.
| Material | Specific gravity |
|---|
| 🟡 Gold | 19.3 |
| ⬛ Black sand (magnetite) | 5.2 |
| ⚪ Quartz sand | 2.65 |
| 🟤 Clay / silt | 1.8 |
Downstream of each riffle bar the flow recirculates, dropping the near-bed water speed — a real low-velocity eddy pocket. A grain only gets trapped there if it has settled to the bed and is moving slower than the pocket can re-entrain it, so taller riffles (bigger pockets) trap more but also silt up with worthless sand; too much water flow sweeps everything through; too little separates nothing. Agitation adds turbulent velocity fluctuations on top of the mean flow — real panning shake — which lofts light, fine grains back into suspension while dense gold stays put (differential mobility, the same physics as kinetic sieving). Motion here runs at roughly 1/8 real speed so the settling — which is genuinely faster than a video frame for a 300 µm gold grain — stays visible.
- Recovery rate — mass of gold trapped behind riffles ÷ mass of gold fed in.
- Concentrate grade — gold's mass share of everything trapped; a well-tuned sluice raises this far above the feed's own gold fraction (2% by count here, much less by mass since gold grains are the smallest).