With a total length of 215 miles (346 km), the Thames is the longest river entirely in England and the second longest in the United Kingdom. It rises at Thames Head in Gloucestershire, and flows into the North Sea at the Thames Estuary via London, the country's capital, where it is particularly deep and navigable; the Thames drains the whole of Greater London. Its tidal section includes most of its London stretch with a rise and fall of 7 metres (23 ft); tides reaching up to Teddington Lock. Along its course are 45 navigation locks with accompanying weirs. Its catchment area covers a large part of South Eastern and a small part of Western England and the river is fed by 38 named tributaries. The river contains over 80 islands. Having both seawater and freshwater stretches, the River Thames supports a variety of wildlife.