The Collaborative International Dictionary of English v.0.48:

Subside \Sub*side"\, v. i. [imp. & p. p. Subsided; p. pr. &
   vb. n. Subsiding.] [L. subsidere; sub under, below + sidere
   to sit down, to settle; akin to sedere to sit, E. sit. See
   Sit.]
   1. To sink or fall to the bottom; to settle, as lees.
      [1913 Webster]

   2. To tend downward; to become lower; to descend; to sink.
      "Heaven's subsiding hill." --Dryden.
      [1913 Webster]

   3. To fall into a state of quiet; to cease to rage; to be
      calmed; to settle down; to become tranquil; to abate; as,
      the sea subsides; the tumults of war will subside; the
      fever has subsided. "In cases of danger, pride and envy
      naturally subside." --C. Middleton.
      [1913 Webster]

   Syn: See Abate.
        [1913 Webster] Subsidence

WordNet (r) 3.0 (2006):

subsiding
    n 1: a gradual sinking to a lower level [syn: settling,
         subsiding, subsidence]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "subsiding":
   at rest, calm, cloistered, collapsing, coming apart, contractive,
   cool, cracking, crumbling, decadent, deciduous, declining,
   declivitous, decreasing, decrescendo, decrescent, decurrent,
   degenerate, deliquescent, descendant, descending, deteriorating,
   diminishing, diminuendo, disintegrating, down, down-reaching,
   downcoming, downfalling, downgoing, downhill, downsinking,
   downward, draining, drooping, dropping, dwindling, ebbing, effete,
   even-tenored, fading, failing, falling, flagging, fragmenting,
   going to pieces, halcyon, hushed, impassive, isolated, languishing,
   lessening, marcescent, moldering, on the descendant,
   on the downgrade, on the wane, pacific, peaceable, peaceful,
   pining, placid, plummeting, plunging, quiescent, quiet, reductive,
   regressive, reposeful, reposing, restful, resting, retrograde,
   retrogressive, sagging, secluded, sequestered, sequestrated,
   setting, sheltered, shriveling, sinking, sliding, slipping,
   slumping, smooth, still, still as death, stillish, stilly, stoic,
   stolid, submerging, tabetic, tottering, tranquil, tumbledown,
   unagitated, undisturbed, unmoved, unperturbed, unruffled,
   unstirring, untroubled, waning, wasting, wilting, withering,
   worsening