The source of the River Thames is in which English county?

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 via the Thames Estuary. On its way, it passes through London, the country's capital, where the river is deep and navigable to ships; the Thames drains the whole of Greater London. Its tidal section, reaching up to Teddington Lock, includes most of its London stretch and has a rise and fall of 7 metres (23 ft). Running through some of the driest parts of mainland Britain and heavily abstracted for drinking water, the Thames's discharge is low considering its length and breadth.