The River Thames (/tɛmz/ TEMZ), known alternatively in parts as the River Isis, is a river that flows through southern England, including London. At 215 mi (346 km), it is the longest river entirely in England and the second-longest in the United Kingdom, after the River Severn. The river rises at Thames Head in Gloucestershire, and flows into the North Sea near Tilbury, Essex and Gravesend, Kent, via the Thames Estuary. From the west it flows through Oxford (where it is sometimes called the Isis), Reading, Henley-on-Thames and Windsor. The Thames also drains the whole of Greater London.