The Collaborative International Dictionary of English v.0.48:

Downfall \Down"fall`\ (doun"f[add]l`), n.
   1. A sudden fall; a body of things falling.
      [1913 Webster]

            Those cataracts or downfalls aforesaid. --Holland.
      [1913 Webster]

            Each downfall of a flood the mountains pour.
                                                  --Dryden.
      [1913 Webster]

   2. A sudden descent from rank or state, reputation or
      happiness; destruction; ruin; as, the senator's
      unrestrained sexual escapades led to his downfall.
      [1913 Webster +PJC]

            Dire were the consequences which would follow the
            downfall of so important a place.     --Motley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

downfall
    n 1: failure that results in a loss of position or reputation
         [syn: downfall, ruin, ruination]
    2: the falling to earth of any form of water (rain or snow or
       hail or sleet or mist) [syn: precipitation, downfall]
    3: a sudden decline in strength or number or importance; "the
       fall of the House of Hapsburg" [syn: fall, downfall]
       [ant: rise]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "downfall":
   Waterloo, atrophy, bane, beating, bouleversement, brash, breakdown,
   burst of rain, cascade, cataract, chute, cloudburst, collapse,
   comedown, conquering, conquest, convulsion, crash, cropper,
   deathblow, debacle, decadence, declension, declination, decline,
   defeat, deflation, defluxion, degeneracy, degeneration,
   degradation, deluge, descending, descension, descent, destroyer,
   destruction, devolution, discomfiture, down, downbend, downcome,
   downcurve, downflow, downgrade, downpour, downrush, downtrend,
   downturn, downward trend, drencher, drop, dropping, drubbing,
   failure, fall, falling, flood, gravitation, gushing rain, headache,
   heavy rain, hiding, inclination, lambasting, lathering, licking,
   mastery, nose dive, overcoming, overthrow, overturn, plash,
   plummeting, pounce, pour, pratfall, problem, prostration, quietus,
   rainburst, rainspout, rainstorm, rapids, ruin, ruination, sabotage,
   scud, smash, soaker, soaking rain, spate, spout, stoop, stumble,
   subdual, subduing, subjugation, subversion, swoop, tailspin,
   thrashing, torrent of rain, trimming, trouble, trouncing, tumble,
   undoing, upheaval, upset, vanquishment, waterfall, waterspout,
   whipping