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]