ETYM Cf. lg. rille a small channel or brook, a furrow, a chamfer, Old Eng. rigol a small brook, French rigole a trench or furrow for water, w. rhill a row, rhigol a little ditch.
A small channel (as one formed by soil erosion).
A very small brook.
Any of several long narrow valleys on the moon's surface.

To flow like a rill