The Collaborative International Dictionary of English v.0.48:

Navigable \Nav"i*ga*ble\, a. [L. navigabilis: cf. F. navigable.
   See Navigate.]
   Capable of being navigated; deep enough and wide enough to
   afford passage to vessels; as, a navigable river.
   [1913 Webster]

   Note: By the common law, a river is considered as navigable
         only so far as the tide ebbs and flows in it. This is
         also the doctrine in several of the United States. In
         other States, the doctrine of the civil law prevails,
         which is, that a navigable river is a river capable of
         being navigated, in the common sense of the term.
         --Kent. --Burrill.
         [1913 Webster] -- Nav"i*ga*ble*ness, n. --
         Nav"i*ga*bly, adv.
         [1913 Webster]

WordNet (r) 3.0 (2006):

navigable
    adj 1: able to be sailed on or through safely; "navigable
           waters"; "a navigable channel"

Bouvier's Law Dictionary, Revised 6th Ed (1856):

NAVIGABLE. Capable of being navigated.
     2. In law, the term navigable is applied to the sea, to arms of the
sea, and to rivers in which the tide flows and reflows. 5 Taunt. R. 705; S.
C. Eng. Com. Law Rep. 240; 5 Pick. R. 199; Ang. Tide Wat. 62; 1 Bouv. Inst.
n. 428.
     3. In North Carolina; 1 M'Cord, R. 580; 2 Dev. R. 30; 3 Dev. R. 59; and
in Pennsylvania; 2 Binn. R. 75; 14 S. & R. 71; the navigability of a river
does not depend upon the ebb and flow of the tide, but a stream navigable by
sea vessels is a navigable river.
     4. By the common law, such rivers as are navigable in the popular sense
of the word, whether the tide ebb and flow in them or not, are public
highways. Ang. Tide Wat. 62; Ang. Wat. Courses, 205 1 Pick. 180; 5 Pick.
199; 1 Halst. 1; 4 Call, 441: 3 Blackf. 136. Vide Arm of the sea; Reliction;
River.