The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
rei-
DEFINITION:
To flow, run. 1. Suffixed zero-grade form *ri-nu-.a.run, runnel, from Old English rinnan, to run, and Old Norse rinna, to run (from Germanic *rinnan, to run, from *ri-nw-an), and from Old English causative ærnan,eornan, to run (from secondary Germanic causative *rannjan); b. Ember day, from Old English ryne, a running, from secondary Germanic derivative *runiz;c.rennet, from Old English *rynet, from secondary Germanic derivative *runita-.2. Suffixed zero-grade form *ri-l-.rill, from Dutch ril or Low German rille, running stream, from Germanic *ril-.3. Suffixed form *rei-wo-.rival, rivulet; derive, from Latin rvus, stream. (Pokorny 3. er- 326.)
vus, stream. (Pokorny 3. er- 326.) 
