Source : Webster's Revised Unabridged Dictionary (1913)

Rill \Rill\ (r[i^]l), n. [Cf. LG. rille a small channel or
   brook, a furrow, a chamfer, OE. rigol a small brook, F.
   rigole a trench or furrow for water, W. rhill a row, rhigol a
   little ditch. [root]11.]
   1. A very small brook; a streamlet.

   2. (Astron.) See {Rille}.

Rill \Rill\, v. i.
   To run a small stream. [R.] --Prior.

Source : WordNet®

rill
     n 1: a small stream [syn: {rivulet}, {run}, {runnel}, {streamlet}]
     2: a small channel (as one formed by soil erosion)