Riffle (r&ibreve_;ff'l), 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.]