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

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "riffle":
   ball up, billow, bollix up, bore, breakers, chop, choppiness,
   chopping sea, chute, cockle, comb, comber, confound, confuse,
   dimple, dirty water, eagre, foul up, fret, fumble, garble,
   gravity wave, ground swell, heave, heavy sea, heavy swell, jumble,
   lift, lop, mix up, muck up, muddle, peak, pi, play hob with,
   popple, rapid, rapids, riff, ripple, rise, roll, roller,
   rough water, sault, scend, scramble, screw up, sea, send, shoot,
   shuffle, snafu, snarl up, surf, surge, swell, tidal bore,
   tidal wave, tide wave, trough, tsunami, tumble, undulation,
   water wave, wave, wavelet, white horses, whitecaps