The River Thames is the longest river entirely in England, rising officially at Thames Head in Gloucestershire, and flowing into the North Sea at the Thames Estuary. It has a special significance in flowing through London, the capital of the United Kingdom, although London only touches a short part of its course. The river is tidal in London with a rise and fall of 7 metres (23 ft) and becomes non-tidal at Teddington Lock. The catchment area covers a large part of South Eastern and Western England and the river is fed by over 20 tributaries. The river contains over 80 islands, and having both seawater and freshwater stretches supports a variety of wildlife.