riffle

A riffle is a transverse bar, groove, or step fixed across the bottom of a sluice box or trough used in placer mining. Its primary function is to trap and hold fine gold particles and other heavy minerals while allowing lighter material to wash through. Riffles create small zones of turbulence and lower velocity where gold and other dense minerals settle, while the faster-moving water and lighter sediment flow downstream.

The term riffle refers both to the trough system itself and to the individual cleats or bars that make up the catching surface. Traditional riffles were wooden slats spaced a few inches apart across the bottom of a sluice box, angled slightly to direct material flow. Modern versions use cast iron, steel, or plastic bars and can incorporate grooves or stepped patterns to increase surface area and improve capture efficiency. The spacing between riffles is critical: too close together and material backs up; too far apart and particles escape.

How riffles work and variants

As ore-bearing sediment enters a sluice box, water velocity and turbulence slow in the depression behind each riffle. Gold settles in these dead zones while lighter material continues downstream. Variations include the expanded metal riffle (a mesh or perforated plate that allows water through but traps sediment), the carpet riffle (fabrics like burlap or rubber placed on the bottom to increase friction), and the undulating riffle (curved steps that optimize flow patterns). Some operations layer multiple materials: carpet over metal riffles increases gold recovery but requires more frequent cleaning and causes greater water backup.

Riffle-based sluicing remains the simplest and most cost-effective method for small-scale gold recovery from alluvial deposits. A single operator can run a small portable sluice box by directing stream water or pumped water through a few ounces of gold-bearing gravel per minute. The method works because placer gold, being 19 times denser than water, segregates rapidly from lighter minerals when flow velocity is properly controlled.

The main operational issue is false concentration: lighter magnetic minerals like magnetite and hematite also settle behind riffles, requiring additional separation steps (pan, shaker table, or magnetic separation) to isolate the gold. Riffles also foul with clay, silt, and organic material, requiring periodic cleaning to maintain efficiency. In high-sediment streams, riffles can become choked in minutes and must be raked clear regularly.