These help to create pools and riffles for fish to spawn.
Long ditches with stone-paved sluices for washing this mineral-bearing material have long been used by the Indians, who also construct stone bars across the beds of the streams to make riffles and hold the deposited grains of gold.
The upper end contains a perforated riddle plate which is placed directly over the riffle box, and under certain circumstances mercury may be placed behind the riffles.
The floor of the sluice is laid with riffles made of strips of wood 2 in.
These were known originally as Hungarian riffles.