The Collaborative International Dictionary of English v.0.48:

Debouch \De*bouch"\, v. i. [imp. & p. p. Debouched; p. pr. &
   vb. n. Debouching.] [F. d['e]boucher; pref. d['e]- (L. dis-
   or de) + boucher to stop up, fr. bouche mouth, fr. L. bucca
   the cheek. Cf. Disembogue.]
   To march out from a wood, defile, or other confined spot,
   into open ground; to issue.
   [1913 Webster]

         Battalions debouching on the plain.      --Prescott.
   [1913 Webster]

   2. (Geog.) To issue; -- said of a stream passing from a gorge
      out into an open valley or a plain.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

debouch
    v 1: march out (as from a defile) into open ground; "The
         regiments debouched from the valley" [syn: debouch,
         march out]
    2: pass out or emerge; especially of rivers; "The tributary
       debouched into the big river"