1. [n]  a small channel (as one formed by soil erosion)
2. [n]  a small stream

RILL is a 4 letter word that starts with R.

1. \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}.
   
   

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

1. One of the first and smallest channels formed by surface runoff.
2. A small erosional channel, usually in parallel where surface waters run on steep unprotected soils.
3. A very small brook or trickling stream of water.