riffle

n. [ CF. G. riffeln, riefeln, to groove. Cf. Rifle a gun. ] 1. (Mining) 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. [ 1913 Webster ]

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. James A. Henshall. [ Webster 1913 Suppl. ]