The Snowbourn arose in the White Mountains under the mountain Starkhorn, flowed through Harrowdale where the Rohirrim capital Edoras was built, and then flowed west until it met the river Entwash in the Eastfold. It also separated Rohan's Eastfold from West Emnet.
During the Return of the King, Theoden asks Grimbold where the Men of Snowbourn are located where he is told none have come (possibly were slain by Isengard) and also through The Lord of the Rings: The Third Age, Berethor's party go there to aid in saving the village being attacked by Uruk-Hai and Dunlending's and throwing out Gríma Wormtongue (explaining why the people of Snowbourn did not go to the Theoden's aid); However, the canonicity of both of these sources can be debated.
|Rivers of Arda|
Adorn | Adurant | Anduin | Aros | Ascar | Brandywine | Brilthor | Brithon | Bruinen | Celebrant | Celebros | Celon | Celos | Ciril | Duilwen | Enchanted river | Entwash | Erui | Esgalduin | Forest River | Gelion | Gilrain | Ginglith | Gladden | Glanduin | Glithui | Greater Gelion | Greyflood | Greylin | Gwathló | Harnen | Isen | Langwell | Lefnui | Legolin | Limlight | Little Gelion | Lune | Malduin | Mering Stream | Mindeb | Mitheithel | Morgulduin | Morthond | Narog | Nenning | Nimrodel | Nunduinë | Poros | Redwater | Ringló | Ringwil | River Running | Rivil | Sarn Gebir | Serni | Sirannon | Siril | Sirion | Sirith | Snowbourn | Taeglin | Thalos | The Water | Withywindle