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. It separated Rohan's Eastfold from West Emnet.
The results were Horaldan, called the Grey Watcher of Snowbourn, a Horse Lord, and Thuriniel, a young woman originally trained to be a spy for Gondor, but whose wood-craft earned her a transfer to Faramir's scouting corps.
Horaldan has come with several Snowbourn riders to answer the call of Éomer: Dunlendings have invaded Rohan and built a fort along the Entwash, fouling the water and raiding the land, and they must be driven off.
To Thuriniel, he mentions that messengers are available in Snowbourn, then pulls Horaldan aside and tells him to keep an eye on the other scout, lest she betray Rohan like the other did.