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]

WordNet (r) 3.0 (2006):

arroyo
    n 1: a stream or brook

U.S. Gazetteer Counties (2000):

Arroyo -- U.S. Municipio in Puerto Rico
   Population (2000):    19117
   Housing Units (2000): 7287
   Land area (2000):     15.041121 sq. miles (38.956324 sq. km)
   Water area (2000):    20.597075 sq. miles (53.346177 sq. km)
   Total area (2000):    35.638196 sq. miles (92.302501 sq. km)
   Located within:       Puerto Rico (PR), FIPS 72
   Location:             17.985289 N, 66.056396 W
   Headwords:
    Arroyo
    Arroyo, PR
    Arroyo Municipio
    Arroyo Municipio, PR