An important river of the Shire, running through both the Westfarthing and the Eastfarthing and on into the river Brandywine. Its source lay northwestward of the village of Needlehole, and early on its course its waters spread to form Rushock Bog before streaming on eastward to form a wide shallow vale, the Water-valley, that extended eastward through the Shire.

From Rushock Bog, the Water flowed on between banks lined by alder trees until it ran beneath the bridge at Hobbiton and turned the wheel of the great Old Mill that stood there. A little downstream, it spread to form a wide Pool at Bywater, a village to which it gave its name. The road from Hobbiton ran along the Pool's southern bank, fringed by a row of trees. Flowing on eastward from Bywater, the Water split briefly into two streams, giving rise to another marshy region northward of Frogmorton. From there it ran on to empty into the Brandywine on the Shire's eastern border, just to the north of the Brandywine Bridge.

During the turbulent times at the end of the Third Age, the Water suffered like the rest of the Shire. The Old Mill at Hobbiton was demolished, and Sharkey's ruffians fouled all the lower waters of the stream, so that filth flowed out into the Brandywine. Many of the trees and houses that ran along the Water's banks were also destroyed at this time, though the later work of the Shire-hobbits, and especially Samwise Gamgee, did much to repair the damage to the Shire and its river.