Runnel (noun)

1. A small stream or brook.
2. A small channel or trench for carrying off water.
3. A narrow groove, furrow, or the like on a surface.

Origin:

Middle english, from old english runnel, from runnan to run.

Examples:

1. The runnel that runs through the village is too shallow for fish to swim in.
2. The water in the runnel was clear and sparkling.
3. The path followed the runnel for a mile, before turning away.
4. The runnel in the rock was deep and dark.
5. The runnel in the sand was a sign of the previous night's rain.