srew-
“to flow, to stream”stream
PIE root meaning to flow or to stream. Source of English "stream," Greek rheîn, and words for flowing water.
Discussion
*srew- is a Proto-Indo-European root meaning "to flow" or "to stream," yielding words for flowing water, currents, and streams.
The root gives Greek rheîn "to flow" and rhéos "stream, current" (whence English rheumatism, rhetoric—originally "flowing speech," diarrhea, and the Rh- prefix in river names like Rhine, though this connection is debated). Latin rivus may also be related, though most scholars derive it from *h₁reyH-.
Old English strēam "stream" (modern English stream) derives from the zero-grade form with the s-mobile prefix (*s-rew-mo-), with Grimm's Law giving *sr > str. German Strom "river, current" and Dutch stroom follow the same pattern. Old Norse straumr yields the Maelstrom compound.
Sanskrit sravati "flows" and srótas- "stream" preserve the root without the s-mobile. Lithuanian sravė́ti "to flow" confirms Balto-Slavic attestation.
The s-mobile in Germanic (stream vs. Greek rhein) is a classic example of this PIE phonological alternation. Modern descendants include English stream, rheumatism, rhythm (disputed), diarrhea, and German Strom.
Notes
Source of English "stream", Greek "rheîn" (to flow). Zero-grade *sru-.