The River Wye (; ) is the fourth-longest river in the UK, stretching some from its source on Plynlimon in mid Wales to the Severn estuary. For much of its length the river forms part of the border between England and Wales. The Wye Valley (lower part) is designated an Area of Outstanding Natural Beauty. The Wye is important for nature conservation and recreation, but is severely affected by pollution. Etymology The meaning of the river's name is not clear. Possibly the earliest reference to the name is Guoy in Nennius' early 9th Century Historia Brittonum and the modern Welsh name is Gwy. The Wye was much later given a Latin name, Vaga, an adjective meaning 'wandering'. The Tithe map references a Vagas Field in both Whitchurch and Chepstow. Philologists such as Edward Lye and Joseph Bosworth in the 18th and early 19th centuries suggested an Old English derivation from wæg, "wave". Description The source of the Wye is in the Welsh mountains at Plynlimon. It flows through or past several towns and villages, including Rhayader, Builth Wells, Hay-on-Wye, Hereford (the only city on the River Wye), Ross-on-Wye, Symonds Yat, Monmouth and Tintern, meeting the Severn estuary just below Chepstow. The lower of the river from Redbrook to Chepstow forms the border between England and Wales. Conservation The River Wye is protected by two Sites of Special Scientific Interest, one covering the Upper Wye (Gwy Uchaf) above Hay-on-Wye, and one covering the Lower Wye (Gwy Isaf) downstream to Chepstow. The criteria for inclusion of the river as an SSSI include geology, topography, flora, mammals, invertebrates, fish and birdlife, as the river and its tributaries constitute a large linear ecosystem. The Lower Wye SSSI is itself divided into seven units of assessment set by Natural England, and administrative responsibilities are shared between the councils of Powys, Herefordshire, Gloucestershire, and Monmouthshire. The Wye abuts a range of other SSSIs in England and Wales, including the Upper Wye Gorge and Lower Wye Gorge. It is also a Special Area of Conservation and one of the most important rivers in the UK for nature conservation. It is an important migration route and wildlife corridor, as well as a key breeding area for many nationally and internationally important species. The river supports a range of species and habitats covered by European Directives and those listed under Schedule 5 of the Wildlife and Countryside Act 1981.