Source : Webster's Revised Unabridged Dictionary (1913)

Arroyo \Ar*roy"o\, n.; pl. {Arroyos}. [Sp., fr. LL. arrogium;
   cf. Gr. ? river, stream, fr. ? to flow.]
   1. A water course; a rivulet.

   2. The dry bed of a small stream. [Western U. S.]

Source : WordNet®

arroyo
     n : a stream or brook