A rivulet or small brook.
A long, narrow channel, typically lined with stone and containing still or moving water.
in common usage a rivulet or brook. In mountaineering, a groove in snow created by a falling cornice. Crossing runnels is slow going. A series of runnels often ends an ascent.
A narrow channel or rivulet of water.
groove in an otherwise featureless area of rock, cause by water erosion (or, in old quarries, by quarrymen drilling and inserting explosives). [Andrew Meldrum