1) Brook 2) Furrow 3) Rill 4) Rillet 5) Rivulet 6) Streamlet 1) Brook 2) Rill 3) Rivulet 4) Small brook 5) Small stream 6) Stream 7) Streamlet 8) Watercourse 
little brook
In common usage a rivulet or brook, in mountaineering, a groove in snow created by a falling cornice. Because crossing runnels is slow going, the prospect of crossing a series of runnels often ends an ascent.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.
• (n.) A rivulet or small brook.
Run'nel noun [ From Run . Confer Rindle .] A rivulet or small brook. « Bubbling runnels joined the sound.» Collins. « By the very sides of the way . . . there are slow runnels , in which one can see the minnows swimming.» Masson. 
A corrugation or trough formed in the foreshore or in the bottom just offshore by waves or tidal currents.
a small stream; brook; rivulet. · a small channel, as for water.