The Collaborative International Dictionary of English v.0.48:

Retreat \Re*treat"\, v. i. [imp. & p. p. Retreated; p. pr. &
   vb. n. Retreating.]
   To make a retreat; to retire from any position or place; to
   withdraw; as, the defeated army retreated from the field.
   [1913 Webster]

         The rapid currents drive
         Towards the retreating sea their furious tide.
                                                  --Milton.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "retreating":
   boat-shaped, boatlike, bowl-shaped, bowllike, cavelike, cavernous,
   concave, concaved, craterlike, cup-shaped, cupped, cymbiform,
   declining, diminishing, dish-shaped, dished, dishing, dishlike,
   dwindling, dying, ebbing, fading, funnel-breasted, funnel-chested,
   funnel-shaped, hollow, hollowed, incurved, incurving, incurvous,
   infundibular, infundibuliform, navicular, naviform, receding,
   retiring, saucer-shaped, scaphoid, scyphate, shrinking, sinking,
   spoonlike, sunk, sunken, waning