The Collaborative International Dictionary of English v.0.48:

Arroyo \Ar*roy"o\, n.; pl. Arroyos. [Sp., fr. LL. arrogium;
   cf. Gr. ? river, stream, fr. ? to flow.]
   1. A water course; a rivulet.
      [1913 Webster]

   2. The dry bed of a small stream. [Western U. S.]
      [1913 Webster]