The Collaborative International Dictionary of English v.0.48:

Runnel \Run"nel\, n. [From Run. Cf. Rindle.]
   A rivulet or small brook.
   [1913 Webster]

         Bubbling runnels joined the sound.       --Collins.
   [1913 Webster]

         By the very sides of the way . . . there are slow
         runnels, in which one can see the minnows swimming.
                                                  --Masson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "runnel":
   adolescent stream, aqueduct, arroyo, beck, bed, bourn,
   braided stream, branch, brook, brooklet, bum, burn, canal, channel,
   creek, creek bed, crick, culvert, donga, dry bed, flowing stream,
   flume, fluviation, fresh, freshet, gill, gulch, gully, gullyhole,
   headrace, irrigation ditch, kill, lazy stream, meandering stream,
   midchannel, midstream, millstream, moving road, navigable river,
   nullah, race, racing stream, river, river bed, riverway, rivulet,
   run, rundle, runlet, sike, sluice, spill stream, spillbox,
   spillway, stream, stream action, stream bed, streamlet, streamway,
   subterranean river, swash, swash channel, tailrace, wadi,
   water carrier, water channel, water furrow, water gap, water gate,
   watercourse, waterway, waterworks