The Collaborative International Dictionary of English v.0.48:

Riffle \Rif"fle\ (r[i^]f"f'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.]

WordNet (r) 3.0 (2006):

riffle
    n 1: a small wave on the surface of a liquid [syn: ripple,
         rippling, riffle, wavelet]
    2: shuffling by splitting the pack and interweaving the two
       halves at their corners
    v 1: twitch or flutter; "the paper flicked" [syn: flick,
         ruffle, riffle]
    2: look through a book or other written material; "He thumbed
       through the report"; "She leafed through the volume" [syn:
       flick, flip, thumb, riffle, leaf, riff]
    3: stir up (water) so as to form ripples [syn: ripple,
       ruffle, riffle, cockle, undulate]
    4: shuffle (playing cards) by separating the deck into two parts
       and riffling with the thumbs so the cards intermix