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 corrugation or TROUGH formed in the foreshore or in the bottom just OFFSHORE by WAVES or TIDAL CURRENTS.

a small stream