The Collaborative International Dictionary of English v.0.48:

Rivulet \Riv"u*let\, n. [Earlier rivolet, It. rivoletto, a dim.
   fr. rivolo, L. rivulus, dim. of rivus a brook. CF. Rival,
   Rite.]
   A small stream or brook; a streamlet.
   [1913 Webster]

         By fountain or by shady rivulet
         He sought them.                          --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rivulet
    n 1: a small stream [syn: rivulet, rill, run, runnel,
         streamlet]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "rivulet":
   adolescent stream, arroyo, beck, bourn, braided stream, branch,
   brook, brooklet, burn, channel, creek, crick, flowing stream,
   fluviation, fresh, freshet, gill, kill, lazy stream,
   meandering stream, midchannel, midstream, millstream, moving road,
   navigable river, race, racing stream, river, run, rundle, runlet,
   runnel, sike, spill stream, stream, stream action, streamlet,
   subterranean river, wadi, watercourse, waterway