Other forms: runnels
A very small stream is a runnel. You might entertain yourself by floating tiny paper boats down the runnel that streams through the woods behind your house.
In Old English a stream was a ryne, and a teeny tiny stream was a rynel, the source of runnel. You could also call it a "rivulet," "channel," or "streamlet." As well as the runnels in nature, branching off from brooks and streams, you might use it to describe someone's perspiring forehead: "It was so hot that there were runnels of sweat running down his face."
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