A rhyne (Somerset), rhine/rhyne (Gloucestershire), or reen (South Wales) (all pronounced /ˈriːn/ "reen"; from Welsh rhewyn or rhewin "ditch") is a drainage ditch, or canal, used to turn areas of wetland at around sea level into useful pasture.
Water levels (and hence the level of the water table) will usually be controlled by a system of sluice gates and pumps, allowing the land to become wetter at times of the year when this will improve grass growth. Rhynes represent an early method of swamp or marsh drainage. Large sections of swampland were surrounded by trenches deep enough to drain the water from the encircled mound and leave the land relatively dry. Regular clearing and dredging is necessary to keep the rhynes clear of debris so that they flow freely.
Rhynes have been used extensively in the United Kingdom, especially on marshy coastal areas such as the Somerset Levels, and the North Somerset Levels. Other examples in England still exist in the Framilode and Saul area of Gloucestershire, where they drain into either the River Severn or Sharpness Canal, and around Easter Compton, South Gloucestershire. In South East Wales, they can be found on the Caldicot and Wentloog Levels. Many of them are still in use today. A large number of rhynes in England and Wales are maintained as part of the water level management operations of internal drainage boards. The rhynes near Wembdon village have some early references, including Fichet's rhyne in 1579 and the Great or Wildmarsh rhyne in 1705.
Some sources claim that rhyne is derived from the Irish word rathin, a diminutive of rath —a circular mound or entrenchment.