The River Thames flows through London before reaching the North Sea.
The Thames rises at Thames Head in Gloucestershire and runs eastward through Oxford, Reading, Windsor and Greater London. It reaches the sea through the Thames Estuary, near Southend-on-Sea, after passing important ports and historic riverside communities.
London grew around a crossing and harbour on the Thames. The river became a major route for trade, migration and defense, helping the city develop into an international capital. Its tidal section, known as the Tideway, extends upstream to Teddington Lock, so much of the London stretch is influenced by the North Sea's tides.
A common mix-up is confusing the Thames with the River Severn. The Severn is the United Kingdom's longest river, while the Thames is the longest river entirely in England. The Thames is also sometimes called the Isis in its upper reaches, especially around Oxford.