h₁reyH-
“to flow, to stream (river)”river
PIE root meaning to flow or to stream as a river. Source of words for rivers, streams, and flowing water.
Discussion
*h₁reyH- is a Proto-Indo-European root meaning "to flow" or "to stream," one of the primary roots for river and flowing water vocabulary.
The root gives Latin rīvus "stream" (whence English river, rival—originally "one who shares a river," and riparian). The laryngeal *H produces the long vowel visible in Latin. Old Irish rian "sea" reflects the Celtic continuation. Sanskrit ríṇāti "lets flow" and rī- "to flow" preserve the verbal root.
In Germanic, the root appears in Old English rīþ "stream" and contributes to river names across northern Europe (the Rhine is sometimes connected to this root, though this etymology is disputed). Gothic runs "a running" may be related through a different ablaut grade.
The root belongs to a rich PIE hydrological vocabulary that includes *srew- "to stream," *h₂ep- "water," and *wed- "water/wet." The careful lexical distinctions between types of flowing water suggest the PIE homeland featured varied river systems.
Modern descendants include English river (via Latin rīvus > Old French riviere), rival, rivulet, and riparian.
Notes
Source of Latin "rīvus" (stream), English "river" via Old French.