The Collaborative International Dictionary of English v.0.48:

Avalanche \Av"a*lanche`\ (?; 277), n. [F. avalanche, fr. avaler
   to descend, to let down, from aval down, downward; ? (L. ad)
   + val, L. vallis, valley. See Valley.]
   1. A large mass or body of snow and ice sliding swiftly down
      a mountain side, or falling down a precipice.
      [1913 Webster]

   2. A fall of earth, rocks, etc., similar to that of an
      avalanche of snow or ice.
      [1913 Webster]

   3. A sudden, great, or irresistible descent or influx of
      anything.
      [1913 Webster]

WordNet (r) 3.0 (2006):

avalanche
    n 1: a slide of large masses of snow and ice and mud down a
         mountain
    2: a sudden appearance of an overwhelming number of things; "the
       program brought an avalanche of mail"
    v 1: gather into a huge mass and roll down a mountain, of snow
         [syn: avalanche, roll down]

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "avalanche":
   abundance, affluence, ample sufficiency, ampleness, amplitude,
   blizzard, bonanza, bountifulness, bountiousness, bumper crop,
   coast, copiousness, crystal, deluge, driven snow,
   embarras de richesses, enough, extravagance, extravagancy,
   exuberance, fertility, flake, flood, flow, flurry, foison,
   full measure, fullness, generosity, generousness, glide, glissade,
   glissando, granular snow, great abundance, great plenty, gush,
   igloo, inundation, landslide, landslip, lavishness, liberality,
   liberalness, lots, luxuriance, mantle of snow, maximum, mogul,
   money to burn, more than enough, much, myriad, myriads,
   numerousness, opulence, opulency, outpouring, overabundance,
   overaccumulation, overbounteousness, overcopiousness, overdose,
   overflow, overlavishness, overluxuriance, overmeasure,
   overmuchness, overnumerousness, overplentifulness, overplenty,
   overpopulation, overprofusion, oversufficiency, oversupply,
   plenitude, plenteousness, plentifulness, plenty, plethora,
   prevalence, prodigality, productiveness, profuseness, profusion,
   quantities, redundancy, repleteness, repletion, rich harvest,
   rich vein, richness, riot, riotousness, scads, shower, sideslip,
   skid, slide, slip, slippage, slither, slosh, slush, snow,
   snow banner, snow bed, snow blanket, snow blast, snow fence,
   snow flurry, snow roller, snow slush, snow squall, snow wreath,
   snow-crystal, snowball, snowbank, snowbridge, snowcap, snowdrift,
   snowfall, snowfield, snowflake, snowland, snowman, snowscape,
   snowshed, snowslide, snowslip, snowstorm, spate, stream,
   subsidence, substantiality, substantialness, superabundance,
   superflux, teemingness, wealth, wet snow