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