The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Astron.) See Rille.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rill \Rill\, v. i.
   To run a small stream. [R.] --Prior.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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