Riffle /(rĭf"f'l)/

Rif·fle

Riffle

n.
  1. A trough or sluice having cleats, grooves, or steps across the bottom for holding quicksilver and catching particles of gold when auriferous earth is washed; also, one of the cleats, grooves, or steps in such a trough. Also called ripple. (Mining)
  2. A ripple in a stream or current of water; also, a place where the water ripples, as on a shallow rapid. [Local, U. S.]
    The bass have left the cool depth beside the rock and are on the riffle or just below it.