The Collaborative International Dictionary of English v.0.48:

Nullah \Nul"lah\, n. [Hind. n[=a]l[=a], fr. Skr. n[=a]la tube.]
   A water course, esp. a dry one; a gully; a gorge; -- orig. an
   East Indian term. --E. Arnold.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

nullah
    n 1: a ravine or gully in southern Asia