Riffle
Riffle (rif"f'l) , noun
[CF. German riffeln, riefeln, to groove. Compare 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.
2.
A ripple in a stream or current of water; also, a place where the water ripples, as on a shallow rapid. [Local, United States]
The bass have left the cool depth beside the rock and are on the riffle or just below it.