rill (n.) Look up rill at Dictionary.com
"small brook, rivulet," 1530s, from or related to Dutch and Frisian ril, Low German rille "groove, furrow, running stream," probably from Proto-Germanic *ril- (source also of Old English rið, riþe "brook, stream," which survives only in dialects), a diminutive form from PIE root *reie- "to run, flow" (see Rhine).